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