[TowerTalk] 160M Antenna puzzlement

[Wes]

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).
>