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[TowerTalk] Multi-band dipoles

Subject: [TowerTalk] Multi-band dipoles
From: K8RI on TT <>
Date: Wed, 20 Apr 2011 22:03:40 -0400
List-post: <">>
There are multi band dipoles such as fan dipoles, trap dipoles, stub 
detuned dipoles, and linearly loaded dipoles.

It's been mentioned that with fan dipoles the lowest frequency band has 
it's band width increased, but on the higher frequency of say 160 and 75 
we find the band width narrows. Remember this is also the second 
harmonic of the 160 segment and the bands tend to be harmonically related.

Is it this function that tends to cause the higher bands to lose usable 
band width?

Some swear the praises of trap antennas, but they are relatively 
inefficient compared to others and it takes some beefy traps to handle 
the legal limit. These traps can see some pretty high voltages at the 
legal limit and when used with a tuner these traps can see some even 
higher voltages.

I'd think stub tuning, or decoupling would be easier to do with ladder 
line than the "fan" approach of parallel wires.

I really don't know much at all about the linear loading aspect.

Then there is the *balun* at the feed point.  Most of the commercial 
multi band wire antennas use a voltage balun at the feed point and for 
75 and even 40. Many rigs require a tuner to cover the whole band. Most 
of these baluns are incapable of handling the legal limit with much of 
an SWR and a tuner will allow the balun to get hit with more power, 
generally cooking it when operated outside it's SWR limits. I think many 
of these are pushed to handled the legal limit PEP, let alone higher 
duty cycle modes even with a low SWR.

There is a lot of equipment out there that is generously rated even when 
it comes to PEP, let alone digital modes.


Roger (K8RI)


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