I've made a magnetic base for a quarter wave whip aerial; different
length rods can be screwed onto it for the 2-metre and 4-metre bands and there is a sliding joint which allows a degree of length adjustment.
There are three ring magnets bolted to the underside of a 200mm diameter steel plate with thin polythene discs, to protect the paintwork, between
the magnets and the roof of the van on which it will be used. The total thickness of the polythene discs, the ceramic magnets and their back
plates is enough to raise the underside of the steel plate about 11mm
above the roof of the van.
The braid of the co-ax feeder is soldered to a tag on the steel plate,
but of course this is not in direct contact with the metal of the van
roof which forms the 'infinite' ground plane. The capacitance between
the steel plate and the van roof is about 120 pf, which has a reactance
of about 18 ohms at 72 Mc/s and half as much at 145 Mc/s. To balance
the feed point I have inserted a 120pf capacitor between the centre
conductor of the co-ax and the connection to the rod elements.
Is this going to cause a shift in resonance that can be corrected by adjusting the length of the elements (one of which is 'infinite' anyway)
or is it liable to upset everything. if the latter, is there a solution
that doesn't involve butchering the van roof?
I've made a magnetic base for a quarter wave whip aerial; different
length rods can be screwed onto it for the 2-metre and 4-metre bands and >there is a sliding joint which allows a degree of length adjustment.
There are three ring magnets bolted to the underside of a 200mm diameter >steel plate with thin polythene discs, to protect the paintwork, between
the magnets and the roof of the van on which it will be used. The total >thickness of the polythene discs, the ceramic magnets and their back
plates is enough to raise the underside of the steel plate about 11mm
above the roof of the van.
The braid of the co-ax feeder is soldered to a tag on the steel plate,
but of course this is not in direct contact with the metal of the van
roof which forms the 'infinite' ground plane. The capacitance between
the steel plate and the van roof is about 120 pf, which has a reactance
of about 18 ohms at 72 Mc/s and half as much at 145 Mc/s. To balance
the feed point I have inserted a 120pf capacitor between the centre
conductor of the co-ax and the connection to the rod elements.
Is this going to cause a shift in resonance that can be corrected by >adjusting the length of the elements (one of which is 'infinite' anyway)
or is it liable to upset everything. if the latter, is there a solution
that doesn't involve butchering the van roof?
On 3/10/25 8:33 AM, Liz Tuddenham wrote:
I've made a magnetic base for a quarter wave whip aerial; different
length rods can be screwed onto it for the 2-metre and 4-metre bands and there is a sliding joint which allows a degree of length adjustment.
There are three ring magnets bolted to the underside of a 200mm diameter steel plate with thin polythene discs, to protect the paintwork, between the magnets and the roof of the van on which it will be used. The total thickness of the polythene discs, the ceramic magnets and their back
plates is enough to raise the underside of the steel plate about 11mm
above the roof of the van.
The braid of the co-ax feeder is soldered to a tag on the steel plate,
but of course this is not in direct contact with the metal of the van
roof which forms the 'infinite' ground plane. The capacitance between
the steel plate and the van roof is about 120 pf, which has a reactance
of about 18 ohms at 72 Mc/s and half as much at 145 Mc/s. To balance
the feed point I have inserted a 120pf capacitor between the centre conductor of the co-ax and the connection to the rod elements.
Is this going to cause a shift in resonance that can be corrected by adjusting the length of the elements (one of which is 'infinite' anyway)
or is it liable to upset everything. if the latter, is there a solution that doesn't involve butchering the van roof?
9ohms isn't much, 18ohms is. But yes, that extra 120pf in series with
the coax center will alter resonance which can be corrected by changing
the length of the whip.
I don't see any "non-invasive" solution, assuming you don't even want a pointy contact screw into the metal because maybe the van is fairly new.
That would also rule out sanding down the thickness of the paint :-)
Another option might be to just live with the asymmetry and use a common
mode choke at the antenna to muffle the imbalance. That would have to be secured against wind, of course, but at least you don't have German
autobahn speeds in the UK.
No chance to bolt to something conductive up there? I guess the roof
rack channels on most modern verhicles aren't conductive anymore.
One thing you might consider is to drill or machine three big holes into
the steel plate and sink the magnets into it so they are flush with the bottom surface. That way the whole steel plate could make surface
contact with the roof (with a thin protective plastic layer in between,
of course). That should increase the capacitance substantially.
On Mon, 10 Mar 2025 15:33:33 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
I've made a magnetic base for a quarter wave whip aerial; different
length rods can be screwed onto it for the 2-metre and 4-metre bands and
there is a sliding joint which allows a degree of length adjustment.
There are three ring magnets bolted to the underside of a 200mm diameter
steel plate with thin polythene discs, to protect the paintwork, between
the magnets and the roof of the van on which it will be used. The total
thickness of the polythene discs, the ceramic magnets and their back
plates is enough to raise the underside of the steel plate about 11mm
above the roof of the van.
The braid of the co-ax feeder is soldered to a tag on the steel plate,
but of course this is not in direct contact with the metal of the van
roof which forms the 'infinite' ground plane. The capacitance between
the steel plate and the van roof is about 120 pf, which has a reactance
of about 18 ohms at 72 Mc/s and half as much at 145 Mc/s. To balance
the feed point I have inserted a 120pf capacitor between the centre
conductor of the co-ax and the connection to the rod elements.
Is this going to cause a shift in resonance that can be corrected by
adjusting the length of the elements (one of which is 'infinite' anyway)
or is it liable to upset everything. if the latter, is there a solution
that doesn't involve butchering the van roof?
Most of the capacitance (11mm distance) will be developed across
the polythene discs.( ~ < 1mm ). Would you be happier if it was
just paint? (~ < 0.2mm )
You can bypass this with wide, soft copper strap, or replace the
plastic with the strap alone, providing it's soft copper and the
magnets actually fit the painted surface's flatness, with no
sharp edges. Copper may harden with use/reuse.
Grease will reduce oxidation, but you've got to keep the mating
interface clean, between and for the duration of installs.
Paint 'fog' can be buffed, if noticeable. Same problem if just
plastic spacers are used. (dirt and vibration)
Resulting 'L'is reduced by strap width and number of bypass
locations; to be ignored considering the poor quality of infinity. Capacitance increased by a factor of 25 (using numbers above).
9ohms isn't much, 18ohms is. But yes, that extra 120pf in series with
the coax center will alter resonance which can be corrected by changing
the length of the whip.
I've made it from an assortment of light alloy tubes and rods (the rods
were machined down between centres on a large lathe so they can slide
into the tubes). There's quite a lot of adjustment but I wonder how
much that will affect the terminating impedance of the co-ax (it can be
tuned to be resistive but it may not not necessarily be 50 ohms).
I don't see any "non-invasive" solution, assuming you don't even want a pointy contact screw into the metal because maybe the van is fairly new. That would also rule out sanding down the thickness of the paint :-)
It's not that new but it is 'tidy' and I would like to keep it that way.
It is also used as a 'stealth' camper, so the mounting has to be easily removeable so as not to attract attention when parking up for the night.
Joerg <news@analogconsultants.com> wrote:
On 3/10/25 8:33 AM, Liz Tuddenham wrote:
I've made a magnetic base for a quarter wave whip aerial; different
length rods can be screwed onto it for the 2-metre and 4-metre bands and >>> there is a sliding joint which allows a degree of length adjustment.
There are three ring magnets bolted to the underside of a 200mm diameter >>> steel plate with thin polythene discs, to protect the paintwork, between >>> the magnets and the roof of the van on which it will be used. The total >>> thickness of the polythene discs, the ceramic magnets and their back
plates is enough to raise the underside of the steel plate about 11mm
above the roof of the van.
The braid of the co-ax feeder is soldered to a tag on the steel plate,
but of course this is not in direct contact with the metal of the van
roof which forms the 'infinite' ground plane. The capacitance between
the steel plate and the van roof is about 120 pf, which has a reactance
of about 18 ohms at 72 Mc/s and half as much at 145 Mc/s. To balance
the feed point I have inserted a 120pf capacitor between the centre
conductor of the co-ax and the connection to the rod elements.
Is this going to cause a shift in resonance that can be corrected by
adjusting the length of the elements (one of which is 'infinite' anyway) >>> or is it liable to upset everything. if the latter, is there a solution >>> that doesn't involve butchering the van roof?
9ohms isn't much, 18ohms is. But yes, that extra 120pf in series with
the coax center will alter resonance which can be corrected by changing
the length of the whip.
I've made it from an assortment of light alloy tubes and rods (the rods
were machined down between centres on a large lathe so they can slide
into the tubes). There's quite a lot of adjustment but I wonder how
much that will affect the terminating impedance of the co-ax (it can be
tuned to be resistive but it may not not necessarily be 50 ohms).
I don't see any "non-invasive" solution, assuming you don't even want a
pointy contact screw into the metal because maybe the van is fairly new.
That would also rule out sanding down the thickness of the paint :-)
It's not that new but it is 'tidy' and I would like to keep it that way.
It is also used as a 'stealth' camper, so the mounting has to be easily removeable so as not to attract attention when parking up for the night.
Another option might be to just live with the asymmetry and use a common
mode choke at the antenna to muffle the imbalance. That would have to be
secured against wind, of course, but at least you don't have German
autobahn speeds in the UK.
My intention is to use it portable but not mobile. The transciever I am designing doesn't lend itself to mobile operation.
No chance to bolt to something conductive up there? I guess the roof
rack channels on most modern verhicles aren't conductive anymore.
There are quite substantial roof bars made from galvanised steel tube
but they have pieces of rubber under the mounting clips, so they don't puncture the paint and make metallic contact with the metalwork of the
van. You can just see the rearmost bar in the picture of the exterior
cowl at:
http://www.poppyrecords.co.uk/Van/vanconversion.htm#COOKER
I had wondered about a large sheet of very thin flexible alloy, which
could be retained by a further 4 magnets, to give much greater
capacitance - but there are two problems with that idea:
1) Stowing the sheet in the van would present problems.
2) For good 2-metre contacts the van needs to be parked on a hilltop
and hilltops tend to be windy. Manipulating the sheet would be
difficult and it may not stay on the roof for very long.
One thing you might consider is to drill or machine three big holes into
the steel plate and sink the magnets into it so they are flush with the
bottom surface. That way the whole steel plate could make surface
contact with the roof (with a thin protective plastic layer in between,
of course). That should increase the capacitance substantially.
That's an idea, I won't be able to do it with the existing plate, but I
do have a bigger piece that might work. My lathe only has 9" radius
swing (with the bed gapped), so I might have to trepan the holes
instead.
The braid of the co-ax feeder is soldered to a tag on the steel plate,
but of course this is not in direct contact with the metal of the van
roof which forms the 'infinite' ground plane. The capacitance between
the steel plate and the van roof is about 120 pf, which has a reactance
of about 18 ohms at 72 Mc/s and half as much at 145 Mc/s. To balance
the feed point I have inserted a 120pf capacitor between the centre
conductor of the co-ax and the connection to the rod elements.
Is this going to cause a shift in resonance that can be corrected by >adjusting the length of the elements (one of which is 'infinite' anyway)
or is it liable to upset everything.
if the latter, is there a solution
that doesn't involve butchering the van roof?
Once again, I'd suggest investing in a NanoVNA if you don't
already have one. They're very handy and will let you
tweak and test your setup without putting your radio at any
risk.
On 3/10/25 7:11 PM, Dave Platt wrote:
[...]
Once again, I'd suggest investing in a NanoVNA if you don't
already have one. They're very handy and will let you
tweak and test your setup without putting your radio at any
risk.
+1
Initially you could just borrow one from another ham. Ultimately I'd get
one (I did). They are particularly useful for the occasional camping
trip. Some day you might want to play with HF antennas and every new
location will be different. With a NanoVNA the tuning procedure will be
much faster, leaving more time for chats on the ham bands. Or to get the steak started :-)
Just an example: When I built a 2m yagi from scrap metal it took me
around 10 trips up and down the ladder to adjust the gamma match, always
with a run back into the house to check the SWR meter. When I built my
220MHz yagi I had a NanoVNA -> one trip up and down the ladder, done.
Ok, so a NanoVNA might cause weight gain ...
looking at the
rear doors I notice very sturdy hinges where the sections on the side of
the van will have a solid chassis contact. That is where hams sometimes
mount antennas over here, especially big HF antennas. Not only would you solve the grounding issue but you'd also have a solid mounting point
without drilling into the vehicle.
For example, you could get another of those hinges from a vehicle
junkyard, have someone weld a piece of steel to it and then mount the
antenna on that. If you ever sell the van you could mount the hinge you
took off.
One thing you might consider is to drill or machine three big holes into >> the steel plate and sink the magnets into it so they are flush with the
bottom surface. That way the whole steel plate could make surface
contact with the roof (with a thin protective plastic layer in between,
of course). That should increase the capacitance substantially.
That's an idea, I won't be able to do it with the existing plate, but I
do have a bigger piece that might work. My lathe only has 9" radius
swing (with the bed gapped), so I might have to trepan the holes
instead.
Or maybe ask a friend who has a bigger lathe.
On Mon, 10 Mar 2025 15:33:33 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
I've made a magnetic base for a quarter wave whip aerial; different
length rods can be screwed onto it for the 2-metre and 4-metre bands and >there is a sliding joint which allows a degree of length adjustment.
There are three ring magnets bolted to the underside of a 200mm diameter >steel plate with thin polythene discs, to protect the paintwork, between >the magnets and the roof of the van on which it will be used. The total >thickness of the polythene discs, the ceramic magnets and their back
plates is enough to raise the underside of the steel plate about 11mm
above the roof of the van.
The braid of the co-ax feeder is soldered to a tag on the steel plate,
but of course this is not in direct contact with the metal of the van
roof which forms the 'infinite' ground plane. The capacitance between
the steel plate and the van roof is about 120 pf, which has a reactance
of about 18 ohms at 72 Mc/s and half as much at 145 Mc/s. To balance
the feed point I have inserted a 120pf capacitor between the centre >conductor of the co-ax and the connection to the rod elements.
Is this going to cause a shift in resonance that can be corrected by >adjusting the length of the elements (one of which is 'infinite' anyway)
or is it liable to upset everything. if the latter, is there a solution >that doesn't involve butchering the van roof?
Most of the capacitance (11mm distance) will be developed across
the polythene discs.( ~ < 1mm ). Would you be happier if it was
just paint? (~ < 0.2mm )
You can bypass this with wide, soft copper strap, or replace the
plastic with the strap alone, providing it's soft copper and the
magnets actually fit the painted surface's flatness, with no
sharp edges. Copper may harden with use/reuse.
Grease will reduce oxidation, but you've got to keep the mating
interface clean, between and for the duration of installs.
Paint 'fog' can be buffed, if noticeable. Same problem if just
plastic spacers are used. (dirt and vibration)
Resulting 'L'is reduced by strap width and number of bypass
locations; to be ignored considering the poor quality of infinity. Capacitance increased by a factor of 25 (using numbers above).
legg <legg@nospam.magma.ca> wrote:
On Mon, 10 Mar 2025 15:33:33 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
I've made a magnetic base for a quarter wave whip aerial; different
length rods can be screwed onto it for the 2-metre and 4-metre bands and
there is a sliding joint which allows a degree of length adjustment.
There are three ring magnets bolted to the underside of a 200mm diameter
steel plate with thin polythene discs, to protect the paintwork, between
the magnets and the roof of the van on which it will be used. The total
thickness of the polythene discs, the ceramic magnets and their back
plates is enough to raise the underside of the steel plate about 11mm
above the roof of the van.
The braid of the co-ax feeder is soldered to a tag on the steel plate,
but of course this is not in direct contact with the metal of the van
roof which forms the 'infinite' ground plane. The capacitance between
the steel plate and the van roof is about 120 pf, which has a reactance
of about 18 ohms at 72 Mc/s and half as much at 145 Mc/s. To balance
the feed point I have inserted a 120pf capacitor between the centre
conductor of the co-ax and the connection to the rod elements.
Is this going to cause a shift in resonance that can be corrected by
adjusting the length of the elements (one of which is 'infinite' anyway)
or is it liable to upset everything. if the latter, is there a solution
that doesn't involve butchering the van roof?
Most of the capacitance (11mm distance) will be developed across
the polythene discs.( ~ < 1mm ). Would you be happier if it was
just paint? (~ < 0.2mm )
You can bypass this with wide, soft copper strap, or replace the
plastic with the strap alone, providing it's soft copper and the
magnets actually fit the painted surface's flatness, with no
sharp edges. Copper may harden with use/reuse.
Grease will reduce oxidation, but you've got to keep the mating
interface clean, between and for the duration of installs.
Paint 'fog' can be buffed, if noticeable. Same problem if just
plastic spacers are used. (dirt and vibration)
Resulting 'L'is reduced by strap width and number of bypass
locations; to be ignored considering the poor quality of infinity.
Capacitance increased by a factor of 25 (using numbers above).
Perhaps a sheet of aluminium cooking foil would be a quick and easy way
to test it.
Joerg <news@analogconsultants.com> wrote:
For example, you could get another of those hinges from a vehicle
junkyard, have someone weld a piece of steel to it and then mount the
antenna on that. If you ever sell the van you could mount the hinge you
took off.
I have never seen or heard of one of these being scrapped in the UK,
they are all exported to third-world counties and converted into lorries
and buses. (The engine and transmission is reputed to outlast three
sets of bodywork.) Perhaps a hole could be drilled through the hinge
strap to take a bolt - that could be removed later and the hole might
not be noticed. I can do my own welding, but that is much harder to disguise.
One thing you might consider is to drill or machine three big holes into >>>> the steel plate and sink the magnets into it so they are flush with the >>>> bottom surface. That way the whole steel plate could make surface
contact with the roof (with a thin protective plastic layer in between, >>>> of course). That should increase the capacitance substantially.
That's an idea, I won't be able to do it with the existing plate, but I
do have a bigger piece that might work. My lathe only has 9" radius
swing (with the bed gapped), so I might have to trepan the holes
instead.
Or maybe ask a friend who has a bigger lathe.
I have a contact in a heavy engineering factory (I sorted out one of
their CNC machines a few years ago) but another approach might be to
drill a lot of smaller holes and buy a lot of smaller magnets. A sheet
of polythene damp-proof course material or adhesive-backed neoprene
sheet across the whole underside would prevent scratching
Joerg <news@analogconsultants.com> wrote:
On 3/10/25 7:11 PM, Dave Platt wrote:
[...]
Once again, I'd suggest investing in a NanoVNA if you don't
already have one. They're very handy and will let you
tweak and test your setup without putting your radio at any
risk.
+1
Initially you could just borrow one from another ham. Ultimately I'd get
one (I did). They are particularly useful for the occasional camping
trip. Some day you might want to play with HF antennas and every new
location will be different. With a NanoVNA the tuning procedure will be
much faster, leaving more time for chats on the ham bands. Or to get the
steak started :-)
Just an example: When I built a 2m yagi from scrap metal it took me
around 10 trips up and down the ladder to adjust the gamma match, always
with a run back into the house to check the SWR meter. When I built my
220MHz yagi I had a NanoVNA -> one trip up and down the ladder, done.
Ok, so a NanoVNA might cause weight gain ...
I have one too. Get the real one (NanoVNA2 v4) and not one of the many knockoffs.
The real ones are sold on Tindie.
Joerg <news@analogconsultants.com> wrote:
[...]
"L" is actually an idea. What if Liz made a series inductor that
resonates with the 120pF and form a series resonant circuit? Of course
it would have to be a diffent inductor if she changes bands and a switch
That was a possibility I had considered but the thought of having to
tune it put me off.
up there in the typical UK weather won't be so great.
I tend to be a fine-weather camper but the weather can change quite
suddenly and the radio would be more likely to be used during an
enforced stay indoor because of a long wet spell.
On 3/11/25 1:55 AM, Phil Hobbs wrote:
Joerg <news@analogconsultants.com> wrote:
On 3/10/25 7:11 PM, Dave Platt wrote:
[...]
Once again, I'd suggest investing in a NanoVNA if you don't
already have one. They're very handy and will let you
tweak and test your setup without putting your radio at any
risk.
+1
Initially you could just borrow one from another ham. Ultimately I'd get >>> one (I did). They are particularly useful for the occasional camping
trip. Some day you might want to play with HF antennas and every new
location will be different. With a NanoVNA the tuning procedure will be
much faster, leaving more time for chats on the ham bands. Or to get the >>> steak started :-)
Just an example: When I built a 2m yagi from scrap metal it took me
around 10 trips up and down the ladder to adjust the gamma match, always >>> with a run back into the house to check the SWR meter. When I built my
220MHz yagi I had a NanoVNA -> one trip up and down the ladder, done.
Ok, so a NanoVNA might cause weight gain ...
I have one too. Get the real one (NanoVNA2 v4) and not one of the many
knockoffs.
The real ones are sold on Tindie.
I got mine at one of the big ham radio dealerships here in the US. Cost
me around $50 plus tax a couple years ago so it has to be the lowest
grade version but it's perfectly fine for tuning antennas and stuff.
Joerg <news@analogconsultants.com> wrote:
On 3/10/25 7:11 PM, Dave Platt wrote:
[...]
Once again, I'd suggest investing in a NanoVNA if you don't
already have one. They're very handy and will let you
tweak and test your setup without putting your radio at any
risk.
+1
Initially you could just borrow one from another ham. Ultimately I'd get
one (I did). They are particularly useful for the occasional camping
trip. Some day you might want to play with HF antennas and every new
location will be different. With a NanoVNA the tuning procedure will be
much faster, leaving more time for chats on the ham bands. Or to get the
steak started :-)
Just an example: When I built a 2m yagi from scrap metal it took me
around 10 trips up and down the ladder to adjust the gamma match, always
with a run back into the house to check the SWR meter. When I built my
220MHz yagi I had a NanoVNA -> one trip up and down the ladder, done.
Ok, so a NanoVNA might cause weight gain ...
I have one too. Get the real one (NanoVNA2 v4) and not one of the many >knockoffs.
The real ones are sold on Tindie.
Cheers
Phil Hobbs
On 2025-03-11 14:58, Joerg wrote:
On 3/11/25 1:55 AM, Phil Hobbs wrote:
Joerg <news@analogconsultants.com> wrote:
On 3/10/25 7:11 PM, Dave Platt wrote:
[...]
Once again, I'd suggest investing in a NanoVNA if you don't
already have one. They're very handy and will let you
tweak and test your setup without putting your radio at any
risk.
+1
Initially you could just borrow one from another ham. Ultimately I'd
get
one (I did). They are particularly useful for the occasional camping
trip. Some day you might want to play with HF antennas and every new
location will be different. With a NanoVNA the tuning procedure will be >>>> much faster, leaving more time for chats on the ham bands. Or to get
the
steak started :-)
Just an example: When I built a 2m yagi from scrap metal it took me
around 10 trips up and down the ladder to adjust the gamma match,
always
with a run back into the house to check the SWR meter. When I built my >>>> 220MHz yagi I had a NanoVNA -> one trip up and down the ladder, done.
Ok, so a NanoVNA might cause weight gain ...
I have one too. Get the real one (NanoVNA2 v4) and not one of the many
knockoffs.
The real ones are sold on Tindie.
I got mine at one of the big ham radio dealerships here in the US.
Cost me around $50 plus tax a couple years ago so it has to be the
lowest grade version but it's perfectly fine for tuning antennas and
stuff.
Probably so. The one I have ($180ish) has a nice low noise floor and
good stability. The main thing I miss compared with a boat anchor VNA
is the calibration doohickeys.
On Tue, 11 Mar 2025 08:55:42 -0000 (UTC), Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
Joerg <news@analogconsultants.com> wrote:
On 3/10/25 7:11 PM, Dave Platt wrote:
[...]
Once again, I'd suggest investing in a NanoVNA if you don't
already have one. They're very handy and will let you
tweak and test your setup without putting your radio at any
risk.
+1
Initially you could just borrow one from another ham. Ultimately I'd get >>> one (I did). They are particularly useful for the occasional camping
trip. Some day you might want to play with HF antennas and every new
location will be different. With a NanoVNA the tuning procedure will be
much faster, leaving more time for chats on the ham bands. Or to get the >>> steak started :-)
Just an example: When I built a 2m yagi from scrap metal it took me
around 10 trips up and down the ladder to adjust the gamma match, always >>> with a run back into the house to check the SWR meter. When I built my
220MHz yagi I had a NanoVNA -> one trip up and down the ladder, done.
Ok, so a NanoVNA might cause weight gain ...
I have one too. Get the real one (NanoVNA2 v4) and not one of the many
knockoffs.
The real ones are sold on Tindie.
Cheers
Phil Hobbs
Cool. We're designing some RF switch thingies and those RF/sinewave
jocks will expect their silly S-parameters.
Is this it?
https://nanorfe.com/nanovna-v2.html
We'll start with some DC-to-3GHz stuff, so this looks right. If we
move on to 40 GHz, we'll probably send it to a test lab.
On 3/10/25 7:11 PM, Dave Platt wrote:
[...]
Once again, I'd suggest investing in a NanoVNA if you don't
already have one. They're very handy and will let you
tweak and test your setup without putting your radio at any
risk.
+1
Initially you could just borrow one from another ham. Ultimately I'd get
one (I did).
Joerg <news@analogconsultants.com> wrote:
For example, you could get another of those hinges from a vehicle
junkyard, have someone weld a piece of steel to it and then mount the
antenna on that. If you ever sell the van you could mount the hinge you
took off.
I have never seen or heard of one of these being scrapped in the UK,
they are all exported to third-world counties and converted into lorries
and buses.
On 2025-03-11, Liz Tuddenham <liz@poppyrecords.invalid.invalid> wrote:
Joerg <news@analogconsultants.com> wrote:
For example, you could get another of those hinges from a vehicle
junkyard, have someone weld a piece of steel to it and then mount the
antenna on that. If you ever sell the van you could mount the hinge you
took off.
I have never seen or heard of one of these being scrapped in the UK,
they are all exported to third-world counties and converted into lorries and buses.
Possibly still worth a look. Rear door hinges are likely to survive a front collision. even a rear collision. Genuine parts are also an option.
I need to do a crash
course in interpreting Smith Charts.
In article <1r8zjnr.1exnyup1tqp24zN%liz@poppyrecords.invalid.invalid>,
Liz Tuddenham <liz@poppyrecords.invalid.invalid> wrote:
The braid of the co-ax feeder is soldered to a tag on the steel plate,
but of course this is not in direct contact with the metal of the van
roof which forms the 'infinite' ground plane. The capacitance between
the steel plate and the van roof is about 120 pf, which has a reactance
of about 18 ohms at 72 Mc/s and half as much at 145 Mc/s. To balance
the feed point I have inserted a 120pf capacitor between the centre >conductor of the co-ax and the connection to the rod elements.
I'm not at all sure that this "balancing" capacitor is going to help
matters more than it hurts. It will probably increase the series (capacitive) reactance of the antenna system as a whole, raising
the SWR and making life tougher on the radio.
It could help you a lot if you can get access to a NanoVNA
course in interpreting Smith Charts.
On 15.3.2025 11.23, Liz Tuddenham wrote:
I need to do a crash
course in interpreting Smith Charts.
The NanoVNA will show the reflection attenuation or VSWR
if you're intimidated by the Smith chart.
The Smith chart shows the both the amplitude and the phase
of the measured reflection. The coordinates are the resistive
and reactive components of the reflection scaled to the system
reference impedance (here: 50 ohms). The coordinate lines are
circles for easy handling of the reflection. Traces of constant
SWR are circles centered on the 50 ohm center point.
The VNA has very small coaxial connectors (SMA), so you'll
need adapters to more conventional feed line connectors.
I prefer adapters with a piece of cable between the connectors,
to protect the small connectors from mechanical stresses.
Jasen Betts <usenet@revmaps.no-ip.org> wrote:
On 2025-03-11, Liz Tuddenham <liz@poppyrecords.invalid.invalid> wrote:
Joerg <news@analogconsultants.com> wrote:
For example, you could get another of those hinges from a vehicle
junkyard, have someone weld a piece of steel to it and then mount the
antenna on that. If you ever sell the van you could mount the hinge you >> >> took off.
I have never seen or heard of one of these being scrapped in the UK,
they are all exported to third-world counties and converted into lorries >> > and buses.
Possibly still worth a look. Rear door hinges are likely to survive a front >> collision. even a rear collision. Genuine parts are also an option.
The fixing screws for those door hinges are torqued up beyond the capabilities of any of the tools I have in my kit.
The ease of mounting
and demounting a magnetic base still appeals to me - and now I have
taken delivery of a VNA, it will be a lot easier to see where I am going wrong and what I need to do to put it right.
Using the existing quarter-wave whip on the cooker top as my 'infinite'
plane (the weather outside is too cold to experiment on the van roof)
has already shown up the folly of a series capacitor. I can now
separate tuning effects from feed-point impedance errors, whereas
previously I was floundering about in the dark. I need to do a crash
course in interpreting Smith Charts.
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