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