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Re: [TowerTalk] 30m Dipole

To: "Keith Dutson" <>,<>
Subject: Re: [TowerTalk] 30m Dipole
From: Jim Lux <>
Date: Thu, 03 Aug 2006 15:48:52 -0700
List-post: <>
At 01:49 PM 8/3/2006, Keith Dutson wrote:
> >As I understand it that just makes the radio happy.  What is the ERP
> >delta between a resonate antenna and a non-resonant tuner-tuned
> >antenna?
>Essentially identical.
>Are you making some assumptions here?

Yes, I was assuming that the resonant and non-resonant antennas were 
"about" the same length and installed in "about" the same place.

>What is I have a perfectly resonant
>coax fed dipole at 3850 KHz at 140 feet and run 200 watts CW.  Now compare
>this to a 30 foot long piece of wire at 20 feet fed directly with a tuner at
>the same frequency.  Is ERP affected in this case?

ERP (but not EIRP) would be the same (except for copper losses in the 
shorter antenna).  Gain would be different (the dipole is 2.15dBi, the 
short wire, closer to 0dBi, because the pattern would be more 
"omni").  And, there would be whatever effects you have from running the 
antenna close to the ground.

Put that 30 foot piece of wire up at 140 ft with the tuner, and then the 
comparison is a bit more fair.  The current will be higher in the 30 ft 
piece of wire, as opposed to the 130 footer, so IR losses will be 
higher.  And, if you compared at a higher frequency, where the coax losses 
are higher, you might wind up with the lower antenna actually being more 
efficient (in a watts radiated compared to watts out of transmitter) than 
the higher one.

But, for basically the same physically sized antenna, whether it's resonant 
doesn't markedly change the gain or pattern, just the impedance at the 
feedpoint.  So, if you have that 3.5 MHz antenna, and it happens to 
resonante at 3.4 MHz or 3.6 MHz, the pattern and efficiency will be about 
the same, assuming your matching network is reasonably low loss (and at the 

If you're depending on the particular resonance (as in a parasitic array, 
like a Yagi), then that's another story, because the different tuning 
changes the phase.



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