Hi!
Anyone had any experience with a G5RV which is NOT symetrical relative
to ground?
Mine is on a sloping section and varies from 3.2 metres AGL at the
western end to 7.5 metres at the Eastern. Although I get a reasonable
match with a single coil Z match tuner I have reason to believe that
there is quite a large electrical imbalance in the system, notably in
the poor rejection of local noise signals.
I came across a Z match circuit on optushome.com.au by VK3YSF which has
a 25 pF variable capacitor from one side of the open wire feeder to
ground. However the article gives no further information about this
component except to call it a "Balance" capacitor.
Has anyone else come across this problem? If so was it a question of
"cut and try" or is there a proper technical approach one can use?
Hoping for some "feedback".
73"s Cliff Wright ZL1BDA

"cliff wright" wrote in message
...
Hi!
Anyone had any experience with a G5RV which is NOT symetrical relative to
ground?
Mine is on a sloping section and varies from 3.2 metres AGL at the western
end to 7.5 metres at the Eastern. Although I get a reasonable match with
a single coil Z match tuner I have reason to believe that there is quite a
large electrical imbalance in the system, notably in the poor rejection of
local noise signals.
I came across a Z match circuit on optushome.com.au by VK3YSF which has a
25 pF variable capacitor from one side of the open wire feeder to ground.
However the article gives no further information about this component
except to call it a "Balance" capacitor.
Has anyone else come across this problem? If so was it a question of "cut
and try" or is there a proper technical approach one can use?
Hoping for some "feedback".
73"s Cliff Wright ZL1BDA


Cliff

Have a look at Lloyd Butler VK5BR's article below as he covers in some
detail the idea behind the balance capacitor and importantly a basic test
arrangement to confirm its effectiveness.

http://users.tpg.com.au/ldbutler/Zbalance.htm

Oh and I'm pleased that someone has look at my web page! I have just moved
to Western Australia and have put a new and slightly improved site due to
the new call. See blow. I will add some additional information regarding the
balance capacitor feature on the Z-match.

http://members.optushome.com.au/vk6ysf/vk6ysf/main.htm

Cheers

Peter VK6YSF formerly VK3YSF

Hi Cliff,

It is often assumed that imbalance is caused by either sloping or the
proximity of objects to one of the legs of a dipole. In my experience it
is far more likely that imbalance is a result of some imbalance in the
tuner or from a feedline that is not running perpendicular to the
antenna over a big enough length.

I normally use a 2 coil Z-match that works quite well, but I have
experimented with the single coil design and also noticed some imbalance.

Since the output coil is floating you would not expect any imbalance,
but the parasitic capacitance between primary and output coil is the
culprit here. That is why the effect is more pronounced at higher
frequencies and relatively high differential impedances.

In the high frequency part of the two coil Z-match the centre of the
primary circuit is connected to ground, so the parasitic capacitance is
far less likely to cause imbalance.

73, Arend

cliff wright wrote:
Hi!
Anyone had any experience with a G5RV which is NOT symetrical relative
to ground?
Mine is on a sloping section and varies from 3.2 metres AGL at the
western end to 7.5 metres at the Eastern. Although I get a reasonable
match with a single coil Z match tuner I have reason to believe that
there is quite a large electrical imbalance in the system, notably in
the poor rejection of local noise signals.
I came across a Z match circuit on optushome.com.au by VK3YSF which has
a 25 pF variable capacitor from one side of the open wire feeder to
ground. However the article gives no further information about this
component except to call it a "Balance" capacitor.
Has anyone else come across this problem? If so was it a question of
"cut and try" or is there a proper technical approach one can use?
Hoping for some "feedback".
73"s Cliff Wright ZL1BDA

cliff wright wrote in news:47a1325a$1
@clear.net.nz:

....
I came across a Z match circuit on optushome.com.au by VK3YSF which has
a 25 pF variable capacitor from one side of the open wire feeder to
ground. However the article gives no further information about this
component except to call it a "Balance" capacitor.

It is no surprise that the Z match is susceptible to a difference in the
magnitude of the voltage on each balanced port wire as described by
Lloyd, VK5BR.

A possible explanation of a certain construction of the Z match circuit
is that the earthy n turns of the main inductor and the output link form
a two wire transmission line.

Looking at Lloyd's article at
http://users.tpg.com.au/ldbutler/SingleCoilVK5BRCct.htm , the two wire
transmission line is 4*pi*62mm in length which represents about 30°
electrical length at 10m. Such a phase delay suggests that lumped
component analysis (eg as an ideal transformer) will probably not
adequately capture the distributed RLGC nature of the two wires.

Analysis as a transmission line offers an explanation of why the currents
flowing in both ends of the 'link winding' or transmission line conductor
into the centre tapped load are not equal. To minimise the high end loss
of balance, the transmission line section needs to be shorter, and an odd
mode Zo closer to the half load Zo (but not necessarily equal to it). TL
analysis also suggests a more symmetric network (with two cascaded TL
sections and the earth connection between the two) that will have more
equal voltage magnitued and phase changes at both ends of the TL.

The balance capacitor introduces another current path which will change
the voltage across the upper half of the load, a crude form of
compensation to try to extend performance.

Owen

pa1are wrote:
Hi Cliff,

It is often assumed that imbalance is caused by either sloping or the
proximity of objects to one of the legs of a dipole. In my experience it
is far more likely that imbalance is a result of some imbalance in the
tuner or from a feedline that is not running perpendicular to the
antenna over a big enough length.

I normally use a 2 coil Z-match that works quite well, but I have
experimented with the single coil design and also noticed some imbalance.

Since the output coil is floating you would not expect any imbalance,
but the parasitic capacitance between primary and output coil is the
culprit here. That is why the effect is more pronounced at higher
frequencies and relatively high differential impedances.

In the high frequency part of the two coil Z-match the centre of the
primary circuit is connected to ground, so the parasitic capacitance is
far less likely to cause imbalance.

73, Arend

cliff wright wrote:

Hi!
Anyone had any experience with a G5RV which is NOT symetrical relative
to ground?
Mine is on a sloping section and varies from 3.2 metres AGL at the
western end to 7.5 metres at the Eastern. Although I get a reasonable
match with a single coil Z match tuner I have reason to believe that
there is quite a large electrical imbalance in the system, notably in
the poor rejection of local noise signals.
I came across a Z match circuit on optushome.com.au by VK3YSF which
has a 25 pF variable capacitor from one side of the open wire feeder
to ground. However the article gives no further information about this
component except to call it a "Balance" capacitor.
Has anyone else come across this problem? If so was it a question of
"cut and try" or is there a proper technical approach one can use?
Hoping for some "feedback".
73"s Cliff Wright ZL1BDA
Hi Arend OM.
Thanks for your remarks. I had forgotten that the single coil Z match
could suffer from that problem, but of course you are quite correct.
Perhaps I might go back to the old 2 coil design yet!
All these queries are based on a very bad QRN problem my neigbouring
Ham and I have had from power lines, even though our local lines are
underground! So anything that keeps the sytem balanced is a potential
help in hearing weak signals.
BTW I did try a 50 pF variable from feeder to ground as shown in the
article I mentioned but it had little or no effect.
73"s Cliff Wright ZL1BDA