matching masts as vertical antennas.
 

Good day all.
Have run into a bit of a problem matching my G5RV's support mast as a
vertical on 20 metres.
The mast is 7.5 metres high and insulated from ground with 4 radials at
ground level but insulated from ground by installation along fence lines.
Obviously a typical 1/4 wave vertical would be ~5.1 metres high, but I
had hopes of base loading the 7.5 metre mast to act as a sort of "short
5/8 wave" vertical on 20. Feed is 50 Ohm coax buried up to the matching box.
After a lot of research both in books and on the web I have found very
little information on the design of an antenna like this.
Anyone had any experience of such an idea?
I have plenty of airspaced inductor stock and TX type capacitors for the
network, and as I usually only run about 150 watts here that shouldn't
be too critical.
Any ideas gratefully received!

73's Cliff Wright ZL1BDA ex G3NIA.

matching masts as vertical antennas.
 

On Wed, 25 Jul 2007 16:20:52 +1200, cliff wright
wrote:

Any ideas gratefully received!

Hi Cliff,

Strip out all the extraneous material like caps and inductors (however
they are applied) and simply drive the vertical against your four
radials. Tell us where it resonates, and give us a few SWR readings
around that point (plus/minus 250 Khz in 50 Khz increments).

Do you have the transmission line choked at the feedpoint?

73's
Richard Clark, KB7QHC

matching masts as vertical antennas.
 

cliff wright wrote in
:

Good day all.
Have run into a bit of a problem matching my G5RV's support mast as a
vertical on 20 metres.
The mast is 7.5 metres high and insulated from ground with 4 radials
at ground level but insulated from ground by installation along fence
lines. Obviously a typical 1/4 wave vertical would be ~5.1 metres
high, but I had hopes of base loading the 7.5 metre mast to act as a
sort of "short 5/8 wave" vertical on 20. Feed is 50 Ohm coax buried up
to the matching box. After a lot of research both in books and on the
web I have found very little information on the design of an antenna
like this. Anyone had any experience of such an idea?
I have plenty of airspaced inductor stock and TX type capacitors for
the network, and as I usually only run about 150 watts here that
shouldn't be too critical.
Any ideas gratefully received!

73's Cliff Wright ZL1BDA ex G3NIA.

Cliff,

I have written an article on unloaded verticals as a multiband antenna,
it is at http://www.vk1od.net/multibandunload...ical/index.htm .

If I scale the model results for a 13m vertical to 7.5m, it suggests that
you would have a base feed point impedance around 150+j300. You could
start by designing a matching network for that sort of impedance and fine
tune the network for low VSWR.

I am not clear on whether you are using this vertical as a mast for
something else. If so, all bets are off.

Owen

matching masts as vertical antennas.
 

"cliff wright" wrote in message
...
Good day all.
Have run into a bit of a problem matching my G5RV's support mast as a
vertical on 20 metres.
The mast is 7.5 metres high and insulated from ground with 4 radials at
ground level but insulated from ground by installation along fence lines.
Obviously a typical 1/4 wave vertical would be ~5.1 metres high, but I had
hopes of base loading the 7.5 metre mast to act as a sort of "short 5/8
wave" vertical on 20. Feed is 50 Ohm coax buried up to the matching box.
After a lot of research both in books and on the web I have found very
little information on the design of an antenna like this.
Anyone had any experience of such an idea?
I have plenty of airspaced inductor stock and TX type capacitors for the
network, and as I usually only run about 150 watts here that shouldn't be
too critical.
Any ideas gratefully received!

73's Cliff Wright ZL1BDA ex G3NIA.

You might try the matching system used on the old "Gotham Vertical". This
uses a tapped coil from the base of the mast to ground. The top of the coil
is tapped down until resonance is found. Coax braid is to ground, and the
coax center conductor is tapped up from the bottom of the coil for best
match.

Repeat the tapping, and measuring for best settings, as there is
interaction. Then you can consider removing unused turns off the coil. Or
you can find the taps that match other bands. This same matching method is
often used on mobile antennas, and may still be in the ARRL handbook.

matching masts as vertical antennas.
 

"Wayne" wrote in
news:cbKpi.3766$7w.1151@trnddc05:

....
You might try the matching system used on the old "Gotham Vertical".
This uses a tapped coil from the base of the mast to ground. The top
of the coil is tapped down until resonance is found. Coax braid is
to ground, and the coax center conductor is tapped up from the bottom
of the coil for best match.

Whilst this system might work for an antenna with a capacitive feedpoint,
will it work for an inductive feedpoint such as this ~3/8 lambda attena?

Owen

matching masts as vertical antennas.
 

"Owen Duffy" wrote in message
...
"Wayne" wrote in
news:cbKpi.3766$7w.1151@trnddc05:

...
You might try the matching system used on the old "Gotham Vertical".
This uses a tapped coil from the base of the mast to ground. The top
of the coil is tapped down until resonance is found. Coax braid is
to ground, and the coax center conductor is tapped up from the bottom
of the coil for best match.

Whilst this system might work for an antenna with a capacitive feedpoint,
will it work for an inductive feedpoint such as this ~3/8 lambda attena?

Owen

Just add enough coil to bring it to 3/4 lambda resonance. Wouldn't that
work?

matching masts as vertical antennas.
 

Wayne wrote:
"Owen Duffy" wrote in message
...
"Wayne" wrote in
news:cbKpi.3766$7w.1151@trnddc05:

...
You might try the matching system used on the old "Gotham Vertical".
This uses a tapped coil from the base of the mast to ground. The top
of the coil is tapped down until resonance is found. Coax braid is
to ground, and the coax center conductor is tapped up from the bottom
of the coil for best match.
Whilst this system might work for an antenna with a capacitive feedpoint,
will it work for an inductive feedpoint such as this ~3/8 lambda attena?

Owen

Just add enough coil to bring it to 3/4 lambda resonance. Wouldn't that
work?

No. Adding a coil won't bring the system to resonance -- you can't
cancel inductive reactance by adding inductive reactance. The fact that
a small coil behaves somewhat like a short piece of wire has fooled some
people into thinking that a large coil acts like a long piece of wire.
It doesn't.

Roy Lewallen, W7EL

matching masts as vertical antennas.
 

"Wayne" wrote in
news:srSpi.4065$9A6.827@trnddc01:


"Owen Duffy" wrote in message
...
"Wayne" wrote in
news:cbKpi.3766$7w.1151@trnddc05:

...
You might try the matching system used on the old "Gotham
Vertical". This uses a tapped coil from the base of the mast to
ground. The top of the coil is tapped down until resonance is
found. Coax braid is to ground, and the coax center conductor is
tapped up from the bottom of the coil for best match.

Whilst this system might work for an antenna with a capacitive
feedpoint, will it work for an inductive feedpoint such as this ~3/8
lambda attena?

Owen

Just add enough coil to bring it to 3/4 lambda resonance. Wouldn't
that work?

I don't really understand exactly what you mean, though I know it is a
popular proposition.

If the antenna feedpoint is around 100+j300 (see my previous posting),
the simplest matching network is an L network, and there are two
solutions to it, but both L networks in this specific case require an L
and a C, and both use practical values of L and C. For example, a 84pf
capacitor in shunt with the feedpoint, then a 2.4uH inductor in series
from the feedline to the feedpoint will deliver that transformation with
reasonable efficiency and with one side of the capacitor grounded.

Can you propose the values of a two inductor L network, or a tapped
inductor that will transform 100+j300 to 50+j0.

Owen

matching masts as vertical antennas.
 

Owen Duffy wrote:
"Wayne" wrote in
news:srSpi.4065$9A6.827@trnddc01:


"Owen Duffy" wrote in message
...

"Wayne" wrote in
news:cbKpi.3766$7w.1151@trnddc05:

...

You might try the matching system used on the old "Gotham
Vertical". This uses a tapped coil from the base of the mast to
ground. The top of the coil is tapped down until resonance is
found. Coax braid is to ground, and the coax center conductor is
tapped up from the bottom of the coil for best match.

Whilst this system might work for an antenna with a capacitive
feedpoint, will it work for an inductive feedpoint such as this ~3/8
lambda attena?

Owen

Just add enough coil to bring it to 3/4 lambda resonance. Wouldn't
that work?


I don't really understand exactly what you mean, though I know it is a
popular proposition.

If the antenna feedpoint is around 100+j300 (see my previous posting),
the simplest matching network is an L network, and there are two
solutions to it, but both L networks in this specific case require an L
and a C, and both use practical values of L and C. For example, a 84pf
capacitor in shunt with the feedpoint, then a 2.4uH inductor in series
from the feedline to the feedpoint will deliver that transformation with
reasonable efficiency and with one side of the capacitor grounded.

Can you propose the values of a two inductor L network, or a tapped
inductor that will transform 100+j300 to 50+j0.

Owen
Thanks Guys!
That has already given me a few ideas.
The G5RV is supported by the mast but it is electrically isolated and
about 300 mm away from the mast itself.
Of course there will be coupling effects but the idea is to tune them
out as much as possible for single band use. Operation is mainly on CW
at the low end of the band.
This then might give me 2 antennas on a rather restricted section, and
an alternative especially for receiving.
Many years ago I got very good results with a pair of 1/4 waves
verticals on a bonded iron roof fed with tapped coax to give a cardioid
pattern. It was a superb 20M rx antenna and I worked a lot of
"antipodean" stations in the western Med area even when the band was
apparently dead back in the 1970's.
Can anyone suggest a good text on matching network design?
Now I'm retired I no longer have easy access to Auckland University's
library for reference and will need to make special arrangements.

73's
Cliff Wright ZL1BDA ex G3NIA

matching masts as vertical antennas.
 

On Wed, 25 Jul 2007 16:20:52 +1200, cliff wright
wrote:

Good day all.
Have run into a bit of a problem matching my G5RV's support mast as a
vertical on 20 metres.
The mast is 7.5 metres high and insulated from ground with 4 radials at
ground level but insulated from ground by installation along fence lines.
Obviously a typical 1/4 wave vertical would be ~5.1 metres high, but I
had hopes of base loading the 7.5 metre mast to act as a sort of "short
5/8 wave" vertical on 20. Feed is 50 Ohm coax buried up to the matching box.
After a lot of research both in books and on the web I have found very
little information on the design of an antenna like this.
Anyone had any experience of such an idea?
I have plenty of airspaced inductor stock and TX type capacitors for the
network, and as I usually only run about 150 watts here that shouldn't
be too critical.
Any ideas gratefully received!

73's Cliff Wright ZL1BDA ex G3NIA.
My approach to this situation would be to download the demo version of
EZNEC, model the vertical with the objective of de terming the needed
reactance for tuning, acquire (or have someone else run their copy) of
TLW (ARRL Antenna Handbook software) and pick the easiest matching
setup for you. If you fabricate the matching device and it is not even
close to matching then you don't have an accurate model of the antenna
in EZNEC.

Four radials will work but not real well.

John Ferrell W8CCW
"Life is easier if you learn to
plow around the stumps"

matching masts as vertical antennas.
 

"Owen Duffy" wrote in message
...
"Wayne" wrote in
news:srSpi.4065$9A6.827@trnddc01:


"Owen Duffy" wrote in message
...
"Wayne" wrote in
news:cbKpi.3766$7w.1151@trnddc05:

...
You might try the matching system used on the old "Gotham
Vertical". This uses a tapped coil from the base of the mast to
ground. The top of the coil is tapped down until resonance is
found. Coax braid is to ground, and the coax center conductor is
tapped up from the bottom of the coil for best match.

Whilst this system might work for an antenna with a capacitive
feedpoint, will it work for an inductive feedpoint such as this ~3/8
lambda attena?

Owen

Just add enough coil to bring it to 3/4 lambda resonance. Wouldn't
that work?

I don't really understand exactly what you mean, though I know it is a
popular proposition.

If the antenna feedpoint is around 100+j300 (see my previous posting),
the simplest matching network is an L network, and there are two
solutions to it, but both L networks in this specific case require an L
and a C, and both use practical values of L and C. For example, a 84pf
capacitor in shunt with the feedpoint, then a 2.4uH inductor in series
from the feedline to the feedpoint will deliver that transformation with
reasonable efficiency and with one side of the capacitor grounded.

Can you propose the values of a two inductor L network, or a tapped
inductor that will transform 100+j300 to 50+j0.

Owen
I spent some time pondering this with EZNEC and a smith chart. No, I can't
come up with an inductive way to match it. I learned somthing
today...thanks!

matching masts as vertical antennas.
 

On Jul 26, 11:55 am, "Wayne" wrote:


I spent some time pondering this with EZNEC and a smith chart. No, I can't
come up with an inductive way to match it. I learned somthing
today...thanks!

A 3/8 wave radiator is easily matched with a single variable cap..
Or should be anyway...
MK

matching masts as vertical antennas.
 

matching masts as vertical antennas.
 

"Wayne" wrote in
news:Ll4qi.15820$U47.4532@trnddc08:

....
Owen
I spent some time pondering this with EZNEC and a smith chart. No, I
can't come up with an inductive way to match it. I learned somthing
today...thanks!

The components for a L matching network may both be L for some range of
loads, but not in the case discussed.

There will also be loads where only a shunt component is required, and
this is often achieved on a short vertical by using less inductive
loading than needed to bring the feedpoint to a purely resistive load.
You can think of it as an L network where the series element is supplied
by the antenna. For example, a short antenna on 3.5MHz with a feedpoint
resistance of say 20 ohms, and a large capacitive reactance can be
inductively loaded to deliver 20-j24.5, and it will require only a shunt
inductor of about 0.9uH to match it to 50 ohms.

(Neither of these are solutions to the original questions, but
interesting sidepaths that show why some configurations work in some
situations.)

Owen

matching masts as vertical antennas.
 

On Jul 26, 5:14 pm, Owen Duffy wrote:
wrote roups.com:


A 3/8 wave radiator is easily matched with a single variable cap..

Details?


Place cap at the feedpoint in series . Viola...
This is fairly common really.. IE: feeding "extended"
Inv L's where the length is appx 3/8 wave.
The usual reason for that is to raise the maximum current
point up off the ground a bit to lower ground losses..
BTW, once you find the best match, and find the cap value
needed, you could replace the variable with a fixed cap.
MK

matching masts as vertical antennas.
 

matching masts as vertical antennas.
 

cliff wright wrote in
:

wrote:
On Jul 26, 5:14 pm, Owen Duffy wrote:

wrote
legroups.com:


A 3/8 wave radiator is easily matched with a single variable cap..

Details?



Place cap at the feedpoint in series . Viola...
This is fairly common really.. IE: feeding "extended"
Inv L's where the length is appx 3/8 wave.
The usual reason for that is to raise the maximum current
point up off the ground a bit to lower ground losses..
BTW, once you find the best match, and find the cap value
needed, you could replace the variable with a fixed cap.
MK



G'Day!
Well I have tried the series cap idea of course first.
It is set up at the moment with a 500 pF TX type variable in the
matching box. However according to my SWR bridge it has quite a high
VSWR and varying the 500 pF had very little effect. And yes, the coax
is OK. 50 Ohm foam dielectric type which I have used up to 70 cM OK.
My phased verticals that I mentioned did use capacitive matching in
this case 1000 pF RX variables. But they were just a little too long.
Unfortunately because of space restrictions I am limited to 4 radials.
Thats one reason why I was trying for more than 1/4 wave.

73's Cliff wright ZL1BDA.


Well, it isn't quite "viola" or even "voila!" is it. MK didn't state the
limitations to his method.

The proposed series capacitor will only achieve a reasonable match if the
series resistance component of the feedpoint is around 50 ohms, and the
reactance is inductive and within range of a practical capacitor.

If indeed the antenna had a feedpoint impedance around 100+j300, the
series capacitor would not achieve a match better than VSWR~=2 (which may
or may not be good enough). Your observation that it was bad (whatever
"quite a high VSWR" means), suggests that it has a higher feedpoint R (it
is unlikely to be lower than 50 ohms), or that your capacitor did not
tune out the inductive reactance (unlikely since you used a 500pF
variable).

If you can't match it with a series cap (and no surprises for me there),
you could always try an L network starting with the values that I
previously gave you.

Owen

matching masts as vertical antennas.
 

Owen Duffy wrote:
cliff wright wrote in
:


wrote:

On Jul 26, 5:14 pm, Owen Duffy wrote:


wrote
glegroups.com:


A 3/8 wave radiator is easily matched with a single variable cap..

Details?



Place cap at the feedpoint in series . Viola...
This is fairly common really.. IE: feeding "extended"
Inv L's where the length is appx 3/8 wave.
The usual reason for that is to raise the maximum current
point up off the ground a bit to lower ground losses..
BTW, once you find the best match, and find the cap value
needed, you could replace the variable with a fixed cap.
MK




G'Day!
Well I have tried the series cap idea of course first.
It is set up at the moment with a 500 pF TX type variable in the
matching box. However according to my SWR bridge it has quite a high
VSWR and varying the 500 pF had very little effect. And yes, the coax
is OK. 50 Ohm foam dielectric type which I have used up to 70 cM OK.
My phased verticals that I mentioned did use capacitive matching in
this case 1000 pF RX variables. But they were just a little too long.
Unfortunately because of space restrictions I am limited to 4 radials.
Thats one reason why I was trying for more than 1/4 wave.

73's Cliff wright ZL1BDA.



Well, it isn't quite "viola" or even "voila!" is it. MK didn't state the
limitations to his method.

The proposed series capacitor will only achieve a reasonable match if the
series resistance component of the feedpoint is around 50 ohms, and the
reactance is inductive and within range of a practical capacitor.

If indeed the antenna had a feedpoint impedance around 100+j300, the
series capacitor would not achieve a match better than VSWR~=2 (which may
or may not be good enough). Your observation that it was bad (whatever
"quite a high VSWR" means), suggests that it has a higher feedpoint R (it
is unlikely to be lower than 50 ohms), or that your capacitor did not
tune out the inductive reactance (unlikely since you used a 500pF
variable).

If you can't match it with a series cap (and no surprises for me there),
you could always try an L network starting with the values that I
previously gave you.

Owen

Yes Owen I reckon you are right!
Tried it out again today and the VSWR is a bit worse than 2:1, not good
enough for me at all.
Now all I need is a good reference work to brush up my complex
impeadnaces that I haven't looked at for about 30 years!
73's ZL1BDA

matching masts as vertical antennas.
 

cliff wright wrote:
Owen Duffy wrote:

cliff wright wrote in
:

wrote:

On Jul 26, 5:14 pm, Owen Duffy wrote:


wrote
roups.com:



A 3/8 wave radiator is easily matched with a single variable cap..


Details?




Place cap at the feedpoint in series . Viola...
This is fairly common really.. IE: feeding "extended"
Inv L's where the length is appx 3/8 wave.
The usual reason for that is to raise the maximum current
point up off the ground a bit to lower ground losses..
BTW, once you find the best match, and find the cap value
needed, you could replace the variable with a fixed cap.
MK




G'Day!
Well I have tried the series cap idea of course first.
It is set up at the moment with a 500 pF TX type variable in the
matching box. However according to my SWR bridge it has quite a high
VSWR and varying the 500 pF had very little effect. And yes, the coax
is OK. 50 Ohm foam dielectric type which I have used up to 70 cM OK.
My phased verticals that I mentioned did use capacitive matching in
this case 1000 pF RX variables. But they were just a little too long.
Unfortunately because of space restrictions I am limited to 4 radials.
Thats one reason why I was trying for more than 1/4 wave.

73's Cliff wright ZL1BDA.



Well, it isn't quite "viola" or even "voila!" is it. MK didn't state
the limitations to his method.

The proposed series capacitor will only achieve a reasonable match if
the series resistance component of the feedpoint is around 50 ohms,
and the reactance is inductive and within range of a practical capacitor.

If indeed the antenna had a feedpoint impedance around 100+j300, the
series capacitor would not achieve a match better than VSWR~=2 (which
may or may not be good enough). Your observation that it was bad
(whatever "quite a high VSWR" means), suggests that it has a higher
feedpoint R (it is unlikely to be lower than 50 ohms), or that your
capacitor did not tune out the inductive reactance (unlikely since you
used a 500pF variable).

If you can't match it with a series cap (and no surprises for me
there), you could always try an L network starting with the values
that I previously gave you.

Owen

Yes Owen I reckon you are right!
Tried it out again today and the VSWR is a bit worse than 2:1, not good
enough for me at all.
Now all I need is a good reference work to brush up my complex
impeadnaces that I haven't looked at for about 30 years!
73's ZL1BDA

Owen I just happened across a reference on page 248 of "Amateur radio
Techniques" by Pat Hawker which I had missed before.
LUckily I have a good stock of "minductor" type coils and hope sometime
next week to try the arrangement at(d) in figure 64.
I have also got a stock of miniductor tapping clamps so it will be easy
to vary the feed point and tuning connections.
With a bit of luck, and some good weather I might have it all set up by
about Friday next.
Many thanks for your suggestions.
73"s Cliff Wright ZL1BDA ex G3NIA.

matching masts as vertical antennas.
 

On Jul 27, 8:21 pm, Owen Duffy wrote:
cliff wright wrote :



wrote:
On Jul 26, 5:14 pm, Owen Duffy wrote:

wrote
legroups.com:

A 3/8 wave radiator is easily matched with a single variable cap..

Details?

Place cap at the feedpoint in series . Viola...
This is fairly common really.. IE: feeding "extended"
Inv L's where the length is appx 3/8 wave.
The usual reason for that is to raise the maximum current
point up off the ground a bit to lower ground losses..
BTW, once you find the best match, and find the cap value
needed, you could replace the variable with a fixed cap.
MK

G'Day!
Well I have tried the series cap idea of course first.
It is set up at the moment with a 500 pF TX type variable in the
matching box. However according to my SWR bridge it has quite a high
VSWR and varying the 500 pF had very little effect. And yes, the coax
is OK. 50 Ohm foam dielectric type which I have used up to 70 cM OK.
My phased verticals that I mentioned did use capacitive matching in
this case 1000 pF RX variables. But they were just a little too long.
Unfortunately because of space restrictions I am limited to 4 radials.
Thats one reason why I was trying for more than 1/4 wave.

73's Cliff wright ZL1BDA.

Well, it isn't quite "viola" or even "voila!" is it. MK didn't state the
limitations to his method.

I wasn't really expecting the feed R to be that high.. If it's 100
ohms,
then yep, the best match would be 2:1. If that's the case, then yes,
he would need to use an L network if he wants the match better than
that.
I never had any trouble using this method, but thinking about it, most
of mine were with 160m inv L's.. Maybe it was the shorter vertical
section that gave me a low enough feed Z to match .
I'd have to model it to see.
MK

matching masts as vertical antennas.