Topband: loaded verticals

[Tom Rauch]

Hi Steve and All,

With a large hat and a reasonably small diameter vertical 
conductor, it doesn't matter much where you place the coil. 
Reactance is so low, a reasonable Q inductor has almost no effect 
on radiating efficiency.

>From Eznec........
50 foot vertical with four #14 25 foot long hat wires.

#14 vertical wire base loaded:
Loop radiation resistance 11.13 ohms
Current at top 77% of current at base
Loss with coil Q of 300 (easily obtainable in modest size coil) with 
100 watts applied is 4 watts 

Same #14 ga vertical wire antenna top loaded:
 
Loop radiation resistance 13 ohms
Current at top 90% of current at base
Loss Q=300 @ 100 watts is 5 watts

Virtually the same performance, since ground losses would offset 
any changes of less coil loss. 

With a 12 inch vertical element and the same wire hat.......
Base loaded:
ad resistance 8.3 ohms (worse)
Current at top 58%
Coil loss 6 watts
 
Top loaded:
Rad Res 12.7 ohms
Current at top 100%
Coil loss 6 watts  

It is best to use a reasonably thin vertical element if you can. It 
makes almost no difference where you put the coil, if the vertical 
element's distributed capacitance is less than the hat's effective 
capacitance.

If I were installing a vertical like this, I'd use a small vertical wire, a 
modestly large hat, and put the coil at the base where it could be 
adjusted and tapped in shunt with the feedline for matching. By 
tapping down on the correct size coil, a perfect match could be 
obtained with a single component, giving maximum efficiency and 
bandwidth and only one part. No one would ever be able to tell the 
ifference in system performance, even with a normal FS meter.

The efficiency difference between an UN-UN and other systems, 
with half-way decent components used, is immeasurable. 
>
In my mobile, I use a single shunt L (with slightly high resonance in 
the antenna) for matching. FS measurements indicate absolutely 
no difference from a shunt C and the antenna below resonance, or 
an Un-Un and the antenna exactly resonant.

One reason people think the coil is inefficient is a small gauge coil 
without good air circulation will get quite hot even when dissipating 
as little as 50 watts.

It is important the coil can dissipate 50 watts or more without 
damage (at the kilowatt power level), even though that amount of 
power loss is only about 0.2 dB! 

I hope this helps dispel some common myths about loading. 
Eznec is a good tool for looking at this stuff.


73, Tom W8JI
w8ji@contesting.com



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