Re: [TowerTalk] Re: Dipole Longer or higher?

[Jim Lux <jimlux@earthlink.net>]

At 10:46 AM 2/15/2005, Bill Fuqua wrote:
> Why do you think the oldtimers used ceramic insulators on both the 
> feedline and
> antenna?
>
> If matched and you have low loss feedline , a short dipole radiates just 
> fine. But, the antenna systems Q will go up drastically. This high Q is 
> the real problem. And just a matter of physics. No way around it.  Along 
> with the expensive components in the antenna tuner to accommodate the high 
> circulating currents and voltages due the high antenna system Q.  You 
> could put inductance at the feed point to nullify the capacitive component 
> of the  antenna's impedance but the Q will still be high and it would only 
> be useful at only one narrow band of frequencies.
Or put the auto tuner at the feedpoint, where it really needs to be.  It 
won't solve the circulating reactive current problem, but it will solve the 
feedline loss problem.

For actual numbers...
Assuming a fairly skinny antenna(l/d=2000), a +/- 20% change in length(or 
frequency) results in a reactive component of some +/- 250-300 ohms (on top 
of the 30-100 ohm resistive component).  The reactance/resistance of the 
antenna ranges from -7 to +2.35.

A moderately thin dipole (l/d=1250) that's 1/4 wavelength long (total... 
1/8th on each side) has an impedance of about 13-713j ohms.  That's a 
fairly large X/R ratio, implying the circulating currents will be about 50 
times the radiated current.

If you assume that your matching network has a Q of say, 200, its 
resistance will be 713/200 or about 3.5 ohms.  Combined with the self 
resistance of the antenna of 13 ohms, you'll lose about 20% of your power 
in the inductor, say, 0.7 dB?  Maybe a bit more, because the antenna isn't 
lossless either.

Jim, W6RMK 


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