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Author Topic: Resurrecting a Cushcraft R-7  (Read 807 times)
K2QB
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Posts: 102




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« on: October 11, 2011, 12:06:42 PM »

Well having never been a vertical person, I am in the process of resurrecting an R-7 to compliment my HF beam. I carefully disassembled the sections and cleaned the mating areas to ensure a good electrical connection. The matching box and lower sections were actually in storage for the past 15 years so the matching network and base insulator are in like new condition. I installed the antenna on a pipe driven into the ground. The base of the antenna is roughly 10 feet off the ground. Now, I connected a random length (~50ft)of 50 ohm coax to the antenna and used an MFJ analyzer to see where it would resonate. Well...no where is the answer. Absolutely off the charts except I did find a null up around 65 MHz. So, I took the analyzer out to the antenna and connected it with a short length (~2 ft) piece of coax and lo and behold it pretty much falls within all the bands except 40 meters where it shows resonance around 6.3MHz. Now I visually checked my original piece of coax, checked the connectors and metered it and everything looks fine. So my question to the vertical gurus out there, is my random piece of coax messing up the readings due to some resonance issue, or can it actually be bad and not look it or act like it when metering it. Next question is assuming it is a coax issue that can easily be remedied by lengthening it or shortening it, any ideas as to why 40 meters is so far off? Not sure therte is enough adjustment on the top section to bring it down enough into the 40 meter band.
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WB6BYU
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Posts: 12985




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« Reply #1 on: October 11, 2011, 01:54:40 PM »

Did you look for dips in the SWR, or for X = 0 on the display?

The first will give you the desired results for any coax length.  The second won't when you have a
random length of coax between the analyzer and the antenna.

If you don't see dips in the SWR that correspond to the expected operating bands, you may have
a problem with your coax, or it could be an issue with common mode currents (so your coax
shield is becoming part of the antenna and throwing off the resonance.)  In that case, you may
see different SWR with a transmitter attached to the end of the coax than you do with the
SWR analyzer.
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K2QB
Member

Posts: 102




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« Reply #2 on: October 12, 2011, 07:11:06 AM »

Thanks for the reply. I was looking for the SWR dips with the analyzer and never did connect the rig. I'll give that a try and see if the SWR on the radio gives me different results. I'm thinking it might as I have used the same piece of coax before for testing other antennas including some VHF antennas and had no problems.
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WB6BYU
Member

Posts: 12985




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« Reply #3 on: October 12, 2011, 08:44:40 AM »

Then it sounds like a coax or connector problem.  Sometimes the plugs and/or sockets can
make intermittent connection - I rebuild several lengths of cable until I discovered that the
inner contact on the socket on my SWR meter was expanded slightly - it worked fine on
standard PL-259 plugs, but was intermittent with crimp-on plugs because the tip of the
center pin was smaller.  Given the symptoms I'd guess you have a connection problem,
either a short or open. 

One test is to install a dummy load in place of the antenna and see if you still get the
same readings from the other end, but again you have to make sure that the problem
isn't in the connector on the antenna itself.
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WA2ONH
Member

Posts: 246




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« Reply #4 on: October 12, 2011, 01:23:42 PM »

Just to have some additional info on hand re: Cushcraft verticals,
found this site/articles:

R7 Vertical...Maintenance and Repair
LINK: http://www.iol.ie/~bravo/r7_vertical.htm

Cushcraft Verticals - TROUBLESHOOTING TRAPS
LINK: http://www.iol.ie/~bravo/Troubleshooting%20Traps.htm

Good Luck!
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73 de WA2ONH dit dit    ...Charlie
----------------------------------------------------------------------------
"No time is ever wasted that is spent LEARNING something!"
K2QB
Member

Posts: 102




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« Reply #5 on: October 13, 2011, 06:19:18 AM »

Thanks for the input fellas. I did visit the various R7 sites on the web and some very valuable info out there. Did a quick test last night. Connected the antenna right to the rig and looked at the SWR. Not too bad except on 30 and 40. 30 is running about 3:1 and 40 is off the meter. So I'm thinking a couple of things. One, maybe a bad connection on the antenna at the 30 meter section and or possibly top hat or counterpoise sections. Two, and I'm leaning towards this seeing the analyzer was way off, current on the feedline due to length and/or routing to the antenna. So ordered some new coax and will install it and wind a common mode choke at the base of the antenna and see what happens. Hopefully this will resolve the 30/40 problem as well and then I can just fine tune it.
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