[TowerTalk] JK Mid Tri 15M Issue

Wed Sep 4 17:15:33 EDT 2024

Drew indicates it is a Mid-Tri, not a Mid-Tri 40, so he would have a difficult time adjusting any 40m feed there. ;-)

The thing that gets me: "Symptom - Both SO2R radios show intermittent received signal strength on 15M only.  The change seems to be about 2 or 3 S units of popping in and out.  SWR is changed little, if at all, during these events.  It sometimes settles down, but turning the antenna usually causes more mayhem.”

Looks like the transmit side is just fine, but problems on receive. That indicates it is some sort of problems outside the antenna itself. I tried a 40m butterfly antenna several years ago that gave me the exact same problems whenever a close-by station was on the air. That antenna proved a poor performer, and when I took it down, the 15m problem went away.

Do you have a close-by station that may be transmitting when you are testing? Might not even be a ham station...

Good luck finding it, Drew, but it just “feels” like something is amiss on the harmonically-related band - 40m.

73,
Jack, W6FB

> On Sep 4, 2024, at 3:29 PM, Richard Thorne <rthorne at rthorne.net> wrote:
> 
> Jack may be on to something here, 40m could very well affect 15m
> 
> A few years ago I built and installed a JK 3040sr which is a big antenna.
> 
> One day I noticed that 40m swr was all over the place for no apparent
> reason.
> 
> I loosened the boom to element clamp on the 40m driven element nearest
> to the tower and turned it 90 degrees.  I climbed down the tower and
> checked the coil.  Sure enough it was loose and needed tightening,
> problem fixed.  I was very lucky it was the element that I could reach
> from the tower.
> 
> Unfortunately you don't have easy access to the 40m elements on the
> Mid-Tri-40.  At least its on a crank up and you have the option of
> renting a small man lift to get to the elements.
> 
> If you haven't already, it might be worth a call to Ken and get some
> advice from him.
> 
> Rich - N5ZC
> 
> On 9/4/2024 2:18 PM, Jack Brindle via TowerTalk wrote:
>> Drew;
>> 
>> Any changes to your 40m antenna(s)?  I have had problems in the past with 40m antennas messing with 15m antenna operation. This one is difficult to chase down, but detuning or removing the 40m antenna fixed the problem.
>> You might check your 40m antennas, not just by SWR, but a physical inspection. Something may be amiss there.
>> 
>> 73,
>> Jack, W6FB
>> 
>> 
>>> On Sep 4, 2024, at 12:47 PM, Drew Vonada-Smith <drew at whisperingwoods.org> wrote:
>>> 
>>> Fellow Hams,
>>> 
>>> I am hoping someone might have an idea on this.  It's a stumper.  With All Asia SSB approaching, I am getting concerned.  The antenna in question is a JK Mid-Tri, a multiband interlaced Yagi using a pair of "transmission line" bars to feed the 10/15/20 driven elements.
>>> 
>>> Symptom - Both SO2R radios show intermittent received signal strength on 15M only.  The change seems to be about 2 or 3 S units of popping in and out.  SWR is changed little, if at all, during these events.  It sometimes settles down, but turning the antenna usually causes more mayhem.
>>> 
>>> Evidence:
>>> 
>>>  *
>>> Both SO2R radios act similarly
>>>  *
>>> In the unlikely event that BOTH my low power band filters are bad, there was no change when bypassing them.
>>>  *
>>> In the event that the remote 2 x 8 switch is a problem, changing ports for the 15M selection made no difference.
>>>  *
>>> Bypassing my triplexer and routing directly to the antenna made no difference.
>>>  *
>>> The 15M specific cable from triplexer to the switch (and all others outside) has been replaced.
>>> 
>>> 
>>> These facts along with rotation stimulating the problem would seem to indicate the antenna itself.  I was finally able to climb yesterday and check the 15M DE mounting to the transmission bus bars.  All seemed corrosion free and tight.  Nevertheless, I applied DeOxit and banged and manipulated the connection to break up any hidden oxide.  No effect.  This is not too surprising as I had never seen a SWR change, but I thought it perhaps possible that the effect was RX only due to a corrosion film layer, as sometimes happens in relays.
>>> 
>>> The only remaining issue I can think of is an intermittent in parasitic elements.  The Yagi is 4 el on 15M, so it is hard for me to imagine that a director going non-resonant due to a bad joint could kill 3 S-units.  It just becomes a non-optimized 3-element Yagi, maybe worth 2-3 dB?  Even if the reflector is somehow intermittent to become shorter and act as a director, which I'd think is the worst case scenario, 3 S-units seems a stretch.  In addition, there are no short tip lengths in the design - all tips are about 2 ft long and use good machine screw fasteners w/nylock, not hose clamps.  But I am left without other theories.
>>> 
>>> Most of the antenna is unreachable so I can't just inspect every joint.  Nothing amiss is visible using good binocs at 24 ft Yagi height (crank-up tower retracted).  I am trying to think about a way I can get audio in the field and perhaps smack every element with a long pole.    I have a 30 ft fiberglass telescoping mast I could use for that, perhaps with my FT-817.
>>> 
>>> If any of you can think of anything else to try, I'd sure love to hear it.  This one is a STUMPER!
>>> 
>>> 73, Drew K3PA
>>> 
>>> 
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>>> 
>>> 
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