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Re: [TowerTalk] 160M Antenna puzzlement

To: towertalk@contesting.com
Subject: Re: [TowerTalk] 160M Antenna puzzlement
From: Wes <wes_n7ws@triconet.org>
Date: Sun, 19 Dec 2021 09:55:03 -0700
List-post: <mailto:towertalk@contesting.com>
Another thought. I'm a little short of space between the vertical part of my L and the tower which supports the far end of the loading wire.  So I feature it; the feedpoint is capacitive and below 50 ohm.  I shunt the feedpoint with an inductor which together with the capacitance forms a step-up L-network.  Results in a good match and a built in static bleed, at the expense of a bit of bandwidth reduction.

Wes  N7WS


On 12/19/2021 8:59 AM, Pete Smith N4ZR wrote:
I may be suffering from a case of too much information, but I'm puzzled by my new 160-meter inverted L, about 60 feet vertical with 8 on the ground radials each about 65 feet long

My old inverted L broke, so I put up a new one recently.  To my surprise, it resonated well above the band, indicating it was about 12 feet short.  Because of uncertainty about the availability of additional height at the top corner,  I temporarily added the additional length at the bottom and loosely coiled it.

Measuring the SWR and R/X at the base of the antenna with my Rig Experts AA-55 Zoom, I now see a nice textbook SWR curve centering on 1820 KHz. Measuring the R and X at the same place, I see R of about 30 ohms and X crossing zero near the same frequency.  That seems a little low for only 8 radials on the ground but...

So now I come into the house.  The feedline is about 75 feet of RG-8X to my tower-mounted antenna switch, and then about 170 feet of Buryflex to the shack.  I'm well aware (in general arm-waving terms) of the impedance transformation effect of feedlines, and even at 160 this one is fairly long, but:

In the shack, I see the same nice SWR curve, centered about 10 KHz lower. The confusion arises with the R and X curves - the R at resonance is a little lower - about 26 ohms - but rises on the upside to about 140 ohms at the high end (around 1920 KHz and then drops steeply to around 12 ohms by 2150 KHz.  The X value, on the other hand, crosses zero at around 1780 KHz, and rises to around 70 ohms at 1870 KHz and then dips sharply, crossing zero again at 1915 KHz.

So, two questions:

1.  Is what I am seeing simply the result of the feedline transformation of R and X, and

2.  If it proves impossible to raise the knee of the inverted L further, can I just leave that small coil of wire in place at the bottom of the inverted L?  It seems to work pretty well  - I worked a number of Europeans last night in the Stew, although it was tough to hear them (I have not yet reinstalled my K9AY loop).


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