TopBand: Some antenna help from you...

[km1h@juno.com]

Hi Bill.
>From your description,  you don't have a clue as to what the REAL
resonant frequency is. It is a common misconception that a 1:1 VSWR
always indicates resonance. But in reality it only indicates that the
instrument is seeing 50 Ohms. This can be purely resistive OR a
combination of resistance and reactance. 

The noise bridge, one with a Xl and Xc scale, plus the usual R is the
easiest way to find resonance. You will probably find the R portion at
some point below 50 Ohms anyway with a vertical. From this point you have
two choices, either trim the antenna length for zero reactance at say
1840 or add C or L in SERIES to compensate, not in shunt. This will then
give you resonance but probably not a 1:1 VSWR...which is no big deal at
160M anyway. 

OK; now that you have wasted a lot of time at the antenna the question
is....will the amp load to full power yet??  All of the big Ameritron
amps use a Pi-L circuit on 80-160M as a method to save money in variable
capacitors; not for TVI as they would like you to believe. This results
in a very narrow matching range but it can easily be customized.

In my ancient AL-1500 manual the 160M fixed loading caps are C13 and C14,
500pf each. They are switched in parallel with the 800pf variable Load
cap. Determine if you need more or less C (probably more). Add another RF
Doorknob in parallel to allow full power loading over the portion of the
band you consider most important; something in the 200-500pf range should
suffice.  This method affects the circuit Q a bit but not enough to be
concerned; you are simply shifting the network transformation range a
bit.
 If for some reason you find that the Load cap is all the way
open.....increasing would result in excessive grid current;  then you
have to reduce the total C in C13-14. DO NOT try and get away with only
one cap; it cannot handle the current. Reducing one cap to 300pf is OK;
if you need less than that it is best to replace both with a pair of
equal value.   

To be honest, I probably wouldn't  bother with the antenna first; simply
rework the amp. Coax cable loss contribution by a moderate VSWR on 160M
is minuscule and may not be worth the effort to bother with. I use a pair
of sloping verticals here with elevated radials on 160M and modifying the
amp works fine for me. As long as I can tune thru resonance to a 250ma
grid current at 800ma plate I know the amp (an old LK-500ZC) is happy and
producing 1200W. 

I hope this is not too long winded and also that it may be of use to
others too. The same amp ideas hold true with other brands and even
straight Pi-Networks.

73........Carl    KM1H

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