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MFJ Dipole whip problem

Aug 2nd 2017, 18:50

K0FRG

Joined: Nov 21st 2016, 01:36
Total Topics: 0
Total Posts: 0
I have a FTdx1200 radio and have been having a problem with a new antenna system. Here are the details.

MFJ 1640t antenna. Portable whip antenna for 40 M that I was using as a base antenna with some results but not great. I thought I would improve on it by making it a dipole. I bought a duplicate antenna from MFJ and also their dipole mount made for this purpose, it is a MFJ 347. I hooked everything up per the instructions using a LMR240 coax cable 25ft long bought from the Ham Radio Outlet. The dipole is mounted about 15ft up on a non metallic pole. When I went to tune the antenna using the FTdx I got strange results. Above 7.300 MHz it nulls out and gives very low SWR. Below 7.300MHz I cannot tune the antenna at all and if I try to transmit at 20watts the radio tells me it is High SWR. I tried adjusting the whips on the two antennas (per the MFJ instructions) and could not get any change in the SWR readings.

I double checked all the connections and they were good. I also tried a suggestion from MFJ and made a balun of 8 turns of coax on a 6" form mounted at the feedpont. This did not show any changes either.

I am not sure what to do next. The reviews and reports about this dipole setup are all good and it should work well as assembled. Any help would be greatly appreciated.
Aug 3rd 2017, 09:55

W1VT

Super Moderator

Joined: Apr 4th 1998, 00:00
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https://www.youtube.com/watch?v=w5YdTarpDHU
A video showing the tuning of a 20M version of what you are trying to do.

You may have a bad piece of coax or connection, though this doesn't really explain why the antenna can't be tuned.

You may want to try using each whip individually as verticals to verify that there are no manufacturing defects. And verify the tuning adjustment of each whip.

Zack W1VT
ARRL Senior Lab Engineer
Aug 5th 2017, 12:08

K0FRG

Joined: Nov 21st 2016, 01:36
Total Topics: 0
Total Posts: 0
W1VT
Thanks for the suggestions.
I took the dipole down and tested both the antennas vertically with 2, 1/4 wave radials. They both exhibited the same problem that I had before. Above 7.300 Mhz they would null and below that high SWR, I even tired two different feed lines to make sure that was not a problem. The only thing I can think at this point is that the antenna is near (within a few feet) of my house which is stucco. I am thinking that the screen mesh in the stucco is causing a problem. Other than that I am stumped and don't have a clue as to what to do next. These antennas should present a reasonable SWR to the tuner in the radio in either dipole or single vertical arrangement.
What else can I try?
Aug 5th 2017, 15:07

W1VT

Super Moderator

Joined: Apr 4th 1998, 00:00
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Perhaps you are misinterpreting the SWR measurement. A lot of modern radios don't transmit above 7300, because that would be transmitting out of band. You may think you have a null, but in reality you may have zero transmit power output and therefore zero reflected power.

http://www.kb6nu.com/what-are-we-doing-wrong/

https://www.reddit.com/r/amateurradio/comments/2qntu4/issue_tuning_40m_hamstick_dipole/


Zack W1VT
ARRL Senior Lab Engineer
Aug 5th 2017, 20:17

K0FRG

Joined: Nov 21st 2016, 01:36
Total Topics: 0
Total Posts: 0
Zack,

Thanks again for the advice.

That makes perfect sense. I thought it was odd that it occurred exactly at 7.300. I will try tuning each antenna separately and be much more careful with my tuning technique and search around a bit on different frequencies.

I bought each antenna at a different time some months apart so I don't think both are defective but probably both are good.


Bob
K0FRG

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