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.

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

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

"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.

"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

"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?

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

"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

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

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"