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cliff wright · August 1st 07, 06:08 AM posted to rec.radio.amateur.antenna

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

cliff wright wrote :

wrote:

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

wrote
oglegroups.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

Many thanks folks!
I tried out Pat Hawker (G3VA's) matching network from "Amateur Radio
Techniques" and it works like a charm. SWR now nearly 1:1 and antenna
much better on receive also.
I find that propogation and QRN here makes it very useful to have both
horizontal and vertical antennas available.
Fortunately I had a bit of left over minductor that turned out to be
just right for the tapped coil and it tunes nicely with about 150pF
of capacitance.
This match is even with the G5RV raised on the mast.
Vy 73's Cliff Wright ZL1BDA ex G3NIA.

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