Topband: shunt-fed bandwidth

[Tom Rauch]

Hi Don,

I hope you don't mind my pointing out something. While your 
suggestions are all good ones, I can site cases where the rules are 
broken..... 

> Something to keep in back of your mind if you're shunt or series feeding
> your tower for 160.    If your antenna has a wonderfully wide bandwidth,
> it may be due to a mediocre ground system. 

Consider a case of a single (or small number of) elevated resonant 
radial. Now the radial's bandwidth (it will be very narrow compared 
to a tower) will actually decrease the bandwidth of the system 
while reducing efficiency. If we add radials, bandwidth will increase 
and yet efficiency increases!

If we shorten that resonant radial(s) and load it with an inductor, the 
effect is bandwidth decreases while efficiency also decreases!

It's a good idea to not pay too much attention to bandwidth as a 
sign of system efficiency unless you really have a feel for what it 
should be with the system you have. My most efficient mobile 
antenna also has the widest bandwidth, it's another "rule breaker"!

So how can we measure efficiency? We can't. All we can do is go 
out some distance (it doesn't have to be a mile, a thousand foot or 
less would be OK) and measure a reference signal in a clear area. 
Don't move that antenna or touch that receiving system, and go 
back and quickly add eight or ten temporary cheap thin wire radials 
as long as you can reasonably make them. If the signals doesn't 
change, you probably are OK.

My system starts to fall off noticeably at about 30 radials, so I use 
60. My tall vertical has 100 radials, but only because the radials 
are LONG (over 200 feet in some directions).

If you have a short vertical, or a close spaced phased array, ground 
losses go up and you might need more radials. Don't just connect 
and disconnect radials to see if they help, because just having 
them there unconnected helps reduce loss to some extent.



73, Tom W8JI
w8ji@contesting.com



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