[TowerTalk] 160M Antenna puzzlement

[Jeff Blaine]

Pete,

The common mode choke - that's how many turns and what material? For 160 
something around 8+ turns & type-31 are the "right" rule of thumb 
answers.  Fewer turns or using (example) a type 43 material will have a 
much less effective capability.  I'm guessing you have the mix & turns 
right because a guy using RG400 probably has looked at the other key 
details as well.

With respect to the transformation, TLW says 245' of RG213 at 1.825 - 
with a 30R/0j load will look roughly like 66 R/20j at the transmitter 
end.  Your mix of 8X/213 won't affect it much as the loss on 160m is 
tokey with either line.  The transformation won't affect the overall SWR 
of course, but it will make the R/Xj combo move around like crazy and 
that is helpful.  The cool part is that adding 50' more of 213 gets you 
roughly 50R/-22j and the inductor would restore a lovely SWR at that 
point.  So getting the SWR tweaked where you want it can be done with a 
lumped element (like an inductor) and some length adjustment.  Or you 
can use an L-net there at either the shack or antenna base.  All those 
will work fine but the details of each solution will vary of course.

The length solution will have less loss associated with it in the ideal 
case - but given the relatively high ground system loss you have, any 
one of these answers will be just fine in practical terms.  I've used 
series inductors & l-nets to tweak antennas before with complete success 
- in fact 2 of the ones here have such running now.  Adjusting the 
length of the ends works fine but may involve more labor in the up - 
measure - down - adjust - repeat thing can make you nuts especially.

Good luck!

73/jeff/ac0c
alpha-charlie-zero-charlie
www.ac0c.com


On 12/19/21 10:42 AM, Pete Smith N4ZR wrote:
> I hope that's not the case, Mike.  There is already a two-toroid 
> common mode choke between the coax and the antenna, wound with RG-400.
>
> 73, Pete N4ZR
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> On 12/19/2021 11:30 AM, Mike Fahmie via TowerTalk wrote:
>>   I'm guessing that the coax run is acting as another radial in your 
>> ground system. You can test this theory by placing a ferrite or 
>> isolator at the antenna end of the coax run.-Mike-WA6ZTY
>>
>>      On Sunday, December 19, 2021, 07:59:34 AM PST, Pete Smith 
>> N4ZR<pete.n4zr at gmail.com>  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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