[TowerTalk] Insights on 30m EDZ - nowhere near what I thought

[Neal Sulmeyer]

resonance is the tendency of a system to vibrate with increasing amplitudes at some frequencies of excitation. These are known as the system's resonant frequencies (or resonance frequencies). The resonator may have a fundamental frequency and any number of harmonics.

Ken,

Do not confuse "resonance" with "matching".  Many educated people make this mistake.  A doublet cut as an EDZ is not resonant at the design frequency.   With open wire (or window line) feedline to a 4:1 balun and the a stub to cancel out the reactance, you match it to your 50 ohm feedline (and radio).  I have several such antennas. Two on 20M, one on 40M and one on 30M.

The length of the open feedline to the 4:1 balun is critical, but can be extended by 1/2 wave multiples (don't forget velocity factor when doing the calculations).  There is no point in extending the (shorted) stub. Also be sure you know the impedance and velocity factor of your open wire feedline.  Don't assume the value.  Measure it!

Also, window or ladder line, is severely impacted by water (rain) adhering to it.  Be sure everything is dry and don't expect the match to hold when it gets wet.  I learned the hard way and now just make my own open wire feed lines.

Neal, K4EA  



On Tue, 02/04/2020 06:05 PM, Ken Bauer via TowerTalk <towertalk at contesting.com> wrote:
> 
I thought this would be easy, but I’ve screwed it up. I’m hoping to get some insights from this group of antenna experts re the easiest way to proceed!  Perhaps someone could model it for me?
> 
> I used the calculator at West Mountain Radio:  http://www.westmountainradio.com/antenna_calculator_zepp.php
> 
> Looking for resonance at 10.120mhz. I ordered supplies from Davis RF: their balanced line / coax center insulator, 14awg solid copperweld and 16awg 19strand copperweld 450 ohm ladder line. I ordered a Balun Designs 4:1 current balun.
> 
> The WMR calculator recommended a matching stub of ladderline 10’ 2.2” and each leg of the antenna as 61’ 3.1”. 
> 
> I thought I would add a half wavelength of ladderline to the stub thinking that this would present the same impedance present at the recommended stub length. Figuring a .91% velocity factor, I added 44.22’ to the stub for a total ladder line length of 54’ 5” between the center insulator and the balun. I temporarily set the antenna legs lengths at 61’ 5” and wound about 18” of overage back on the antenna wire leg towards the center insulator on each side.
> 
> I hoisted this up to about 70’ , pretty much in the clear, other than some pine branches at each end. 10’ of RG-213 to my RigExpert AA-55 Zoom dangling at about eye level.
> 
> The SWR plot  shows a dip at 10.9mhz w SWR of 3.7:1  and a dip at 13.02mhz w SWR of 2.7:1.
> 
> Thanks in advance for any insights!
> 
> 73,
> Ken WC6Y
> 
> 
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