TopBand: Some antenna help from you...

[W8JITom@aol.com]

Karl,

You waste a lot of space where you could be helpful 
being "Karl".

In a message dated 97-02-18 10:06:17 EST, you write:
>
>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. 

That statement is totally incorrect. 

There is NO conbination of reactance and resistance that gives
a 1:1 SWR except when reactance is zero.

A 1:1 VSWR always indicates ZERO reactance, and 50 ohms
(with a 50 ohm meter). To the best of my knowlege the lowest SWR
(assuming the meter tunes past resonance) ALWAYS occurs where
reactance is zero.

>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.

The Pi L circuit was selected to give improved matching range,
since room was not available for a larger capacitance. It costs more
money than a larger capacitor, but saves space. The reason it was
used was to save space, not money.

> 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.   

>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.

Karl's suggestions are basically correct, except you will wind up with
a PA designed to work into ONE antenna. At my station, I match the
antennas to 50 ohms, not the PA to each antenna. That way when you
change antennas or PA's you don't need to re-work the tank cirrcuit.

If you think about it, matching the antenna makes MUCH more sense
than customizing the PA to an antenna.

Be aware the loading capacitor in the AL-1500 is critical for current rating
AND voltage. This is true of ANY pi-L. Even 850 series transmitting
capacitors might produce heating and drift problems. The padding capacitor
must be a high current low drift capacitor and NOT some piece from the
junkbox
(unless you are a lucky guy). Many doorknobs won't even work.

73 Tom

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