|To:||"Mel Martin" <email@example.com>, <firstname.lastname@example.org>|
|Subject:||Re: [TowerTalk] SteppIR 40/30|
|From:||Jim Lux <email@example.com>|
|Date:||Tue, 04 Nov 2003 13:43:03 -0800|
At 04:17 PM 11/4/2003 -0500, Mel Martin wrote:
I'm not sure that's necessary... they could use a Hi-Q loading coil... this would work well since bandwith isn't the issue it is with a more conventional shortened 40... it wouldn't have to be much bigger than a 20 meter antenna...
The tricky thing with a shortened element and a loading coil is the increased losses. Not so much in the coil, but because the short element will have a lower radiation resistance, so resistive loss will be a larger fraction of the total load impedance. The reactive power circulating between coil and capacitive antenna will also lead to more IR losses. On the other hand, it IS a multi-element array, so maybe it all comes out in the wash... The full size element array would have some amount of reactive currents circulating just to excite the passive elements.
Taken to an extreme, it starts to look like a compact loop antenna, where the capacitor which tunes out the reactive component is essentially lossless, but the resistance of the element is very significant compared to the milliohm Rr of the loop.
However, a half length element (as in using a 20m element on 40m) is still going to have a fairly decent radiation resistance (roughly 1/4 that for the full size element). The gain will also be slightly less, because the short dipole has a pattern that is more isotropic than the 2.15dBi dipole upon which the standard beam is based.
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.
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