Help: Antenna won’t tune

[Anonymous]

Hi,

KD8WMJ here.  I am an Internet member as I live just  a little to far to attend meetings.  Although that has changed now with virtual meetings.

I recently bought into a HOA that does not permit antennas.  I know, why did I do that?  However I did build a “flagpole” antenna that checks out nicely on an analyzer.  Details: OCF dipole, tuner directly at base, then ferrite feedline choke, 49.5 ft coax, ferrite feedline choke, then transceiver.

On 40 meters the analyzer says SWR of about 15:1, which while high is well within the range of the MFJ939 tuner.  On 20 meters shows about 1:1, 17 meters about 5:1, 15 meters about 6:1, 12 about 5:1, and 10 meters about 4:1.

So, here is the problem, the tuner will NOT tune the antenna on 40 meters or on 15 meters.  Effortlessly tunes the other bands.

Any suggestions as to why, and how to overcome the problem.

[Hamilton]

Hello Bill,

There are many possibilities that could cause the SWR to be high on several bands. A sketch of the antenna design including the  antenna wire and feed lines with their rough dimensions would help nail down a solution.

It is possible the antenna itself is a low impedance (Z) on 40m and the coax is around 1/4 wave (or odd multiples of a 1/4 wave) long which would transform the ant’s low Z to  a High Z at the tuner. It can also go the other way where the antenna is high Z on 40m and the coax can transform it to a very low Z. I suspect something in between these possibilities. One quick test is to insert a length of cable between the antenna and the tuner. For 40m I would try 25′ if it is available, if not use what is handy.

Performance on 15m and 40m are often related due to their 3:1 frequency difference. For example 1/4 wave long on 15m is 3/4 wave long on 40m. 3/4 and 1/4 have the same affect…

Another factor is the actual Z at the tuner. The 939 is a L tuner with a series inductor  and a shunt capacitor making the shape of an L. There are impedance values that cannot be tuned with this combination. While the 939 can place the shunt C before or after the inductor giving it the capability to tune a wider range of impedances, it still doesn’t cover the full range of possibilities. Once again a length of cable between the antenna and the tuner might transform the Impedance to something the 939 can tune.

I would also test the affect of removing the choke on the shack end of the feedline. The choke could make the shack end of the feed-line appear to be high Z which when rotated back down a 3rd of a wavelength (50′ on 40m) to the tuner it may affect the tuner’s anility to tune.

There may be issues with how the feedline is routed to the feed point on the mast. A sketch would help.

It’s nice you got a flagpole up.  Have you been chasing the 13 Colonies special event stations?

Hamilton K1HMS

[Anonymous]

Hi Hamilton,

Thanks for your response.  Here is a link to the antenna I made.

I did not use a Harbor Freight pole because I already had an old pole anyway.  The dimensions are the same though.  The author of the video is John Portune who has been published in various “ham” publications, so I expect the antenna is good.  I have spoken to one of the commercial manufacturers of flagpole antenna and Mr Portune helped them develope a commercial version of his antenna.

I made the antenna here in RI to take to my HOA restricted place in FL.  I have not tried anything with the antenna except to analyze it and try to tune.  I have made a contact about 13colonies using my home QTH antenna.

You probably have hit on the problem because the feedline I am using here in RI is 49.5 ft long.  As soon as I can get some more coax I will make an extension and try it.  In the meantime I will try removing the choke on the receiver end.  I will also run the analyzer again and record the impedance values.

I certainly do appreciate your efforts.  I really don’t know anything about the info you give about impedance changing, reflecting, etc., but now that I have it in front of me I will study it and hopefully get to understand it.  I have read about some of it on Internet sources, but different refences somehow seemed to contradict each other.

Thanks again and 73,

Bill

KD8WMJ

[Author]

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