Topband: Dual Band Inv L - 2 Points to clarify

[Tom Rauch]

Hi Brian,

> I forgot to mention that my feedline is 450 ohm ladderline to
> a HI-Q 4:1 Balun by VGE  and then 3' of Coax to the radio.

Feeding an almost perfectly unbalanced antenna, like an inverted L 
with a good ground system, with a balanced line is not a good idea.

While you can tune the antenna on a couple bands, the feedline 
losses are probably much higher than you might ever expect. The 
feedline also radiates, contributing to the amount of wasted energy.

There are two important points to remember.

1.) The bulk of losses in a feedline on low frequencies are I^2 R 
losses in the conductors. 

2.) When the line is shorter than 1/2 wl or more, standard loss 
tables don't apply! 

I'd be especially concerned if the ladder line you have has small 
conductors, like #18 or so. It can eat you alive on losses. 

You could probably greatly reduce system loss on 160 meters by 
shunting the feedline with a hi-Q inductor that cancels the 
reactance on 160 meters. At least then the feedline won't be part of 
the power factor correction system. The inductor could probably 
stay in line for 80 meters without ill effect, since it is just shunting 
the feedline at the antenna. And if it were my system, I'd use some 
sort of choking balun at the antenna.  
 
> Now I know the tuner should be at the antenna and not in the shack but my
> question is: isn't it better to trade off the TOA angle to a possible
> power loss due to a mismatch at the antenna?? Put it this way - would you
> rather be putting out full power at a TOA of 85 degrees or half as much
> power at 30 degrees?

TOA is probably one of the most misused things modeling 
programs present. TOA by itself is meaningless. What you need to 
do is pick a system that has maximum FS at your target wave 
angle without considering TOA at all. 

In my opinion, many simple modeling programs under-estimate the 
FS at low angles from a vertical because the distance along the 
ground to the point of calculation is so far. I don't think the simple 
programs we use are all that accurate for low angle vertically 
polarized radiator calculations.

I'd put more stock in on-the-air performance than predictions of 
performance at low angles.

73, Tom W8JI
w8ji@contesting.com


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