[TowerTalk] Unusual 160m. L behavior

Jim Brown jim at audiosystemsgroup.com
Mon Feb 18 20:31:34 EST 2019


Yes, pretty unusual.  Some WAGs, and some questions.

Are the radials grounded to a driven rod at the base? This is important 
for lightning, but is not great for antenna performance.

Is 33 ft the limit of what you can do, or is it some number you picked 
(maybe from the old ARRL Handbook table I quoted in my 160M app note)?

Are these on-ground or elevated radials?  If elevated, by how much?

If on ground, I'd try to make each of them as long as possible, and I'd 
add as many as I could.

Is it possible that the work you did connecting the new radials 
disturbed the existing ones?

N6LF has observed that a radial that is longer than a quarter wave 
results in increased loss because current has to be zero at the end and 
will have a peak greater than the feedpoint current a quarter wave from 
the end. Also, note that on-ground radials are lengthened by a velocity 
factor -- on 160M, an on-ground quarter wave is around 100 ft (I say 
"around" because it depends on the soil).  We can measure this by 
disconnecting a pair of them and measuring the pair as a dipole).

Rudy also observes that 1) elevated radials have the lowest loss if they 
are slightly shorter than resonance and 2) if they are all the same 
length and height.

Is it possible that your Z measurement includes the effects of the tower 
as a parasitic element?

Is the Z you measure at the new resonant frequency, or at the old 
resonant frequency?

Is it possible that you're getting RF from a local AM BC station into 
the analyzer?  If so, and if the station goes off the air or reduces 
power at night, try measuring again after dark.

Does the SWR indicator in your rig or in the shack roughly agree with 
SWR at the feedpoint if you make a point-by-point plot every 20-25 kHz 
or so of the in-shack reading?  Is apparent resonance at the same point?

How are you defining resonance -- by zero reactance, or min SWR?

Do you have a serious ferrite choke at the feedpoint so that the coax 
isn't acting as a radial? Obviously won't show up with the analyzer at 
the base, but it will in the shack.

Like I said, some WAGs. :)

73, Jim K9YC

On 2/18/2019 4:50 PM, RVZ via TowerTalk wrote:
> Hi Guys,
>
> I'd appreciate your thoughts on why I'm experiencing unusual behavior on my 160m L.  Correct me if I'm wrong, but I believe that typically when radials are added to a Vertical the impedance goes down, and less radials usually means a higher impedance.  My Inv L is just the opposite: with 8 radials (2 at 135' and 6 at 33') the impedance is 50 ohms.  With 16 radials (2 at 135' and 14 at 33')
> the impedance jumps to 85 ohms.  And not only does the impedance increase with the number of radials, so does the resonant frequency increase, as it jumped from 1825 khz to 1875 khz.
> Seems like mighty weird behavior because I've never seen much of a change in resonant freq. before when adding radials.
> Note: measurements are at the base of the antenna with a AA230PRO.  The Inv. L is 60' vertical and 75' horizontal, and spaced 12' away from a Rohn 45 tower with a SteppIR Yagi top mounted.  Soil was dredged from the bottom of a saltwater river.





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