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Thread: Cushcraft R7 issues

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  1. #1

    Default Cushcraft R7 issues

    I have limited space for antennas, and have been using an MA6V for 10-20. 40 has been problematic. I just put up a used R7, which gives me the addition of 30 and 40 meters.

    I assembled it according to the manual, and tested the traps with my MFJ 259. They appear to be OK.

    10-20 meters are fine and have manageable SWRs. However, on 30 the best I can get is about 4:1 and on 40 it is 8:1. With the internal tuner on the K3, I can get out on all bands, but think that the high SWR is a problem, at least for efficiency.

    Are there any R7 users out there who have had similar SWR problems? I have found many sites with tips on repairing r7s, but none on installation issues.

    73s

    Randy K6CRC

  2. #2
    Join Date
    Dec 2005
    Location
    Sunnyvale, CA
    Posts
    241

    Default

    use your 259 to sweep around 10.100 and find out where the lowest swr is. then move the 30m trap in or out to get it on the right frequency. I bet the 40m will work after that. but you can also adjust the top two sections to select the part of the 40m band you want, CW or Phone.

    I am curious to hear you got the coils to work on the 259. I have a 269 and have never ever gotten the dip coils to work.

  3. #3
    Join Date
    Jan 2000
    Location
    Clearwater, FL
    Posts
    4,981

    Default

    I've used the R-Series of verticals for years, including the R-7. As mentioned, find out WHERE on 30 and 40 meters the antenna shows the lowest SWR. You can then adjust the antenna as necessary. First adjust 30 meters, then 40.

    One issue I had with the R-7/8 is the last installation I had was not ideal. It was on a chimney that had a metal cap on top (the kind to prevent critters, etc from getting into the chimney). This SERIOUSLY detuned the antenna, which wasn't as much a problem on 10 through 20 but it greatly affected 30 and 40 meters. I raised the antenna up higher on the chimney which helped, but didn't completely resolve the issue.

    ...............Bob
    ex-W4DFW Ham since 1970. ARRL Life Member and Volunteer Counsel

  4. #4

    Post

    You description indicates that the problem with your R7 antenna is at or above the 20 meter trap.

    1. The coaxial capacitor poriton of the trap may require adjustment, AFTER you understand where it is resonant (tuned to).
    IF you have read the Troubleshooting Traps web page, then you understand that you may have a hole in the polypropelene dielectric (tubing) OR a break in
    the heat-shrink insulation that permits water penetraiton.


    2. The X-hat must be properly installed.

    3. The trap locations on the main vertical mast, must be as shown in the R7 manual (usually alternating sides) -- to minimize interactions between these traps.

    4. Cushcraft recommends a 10 to 12 foot mast when ground mount, so that the 7 counterpoise wires (HV at tips) are above people and reduces ground interactions.

    w9gb
    Nullius in verba

  5. #5

    Default Followup on R7- additional info

    Thanks to all for assistance. Here is some additional notes, based on the comments in this thread.

    The R7 is on a ground mounted pole attached to the roof support. It is 4ft from the wood box chimney, which hold up a zero clearance fireplace flue. There is a sheet metal cap on the top, about 2ft by 3ft. The radials are not touching the box, and are about a foot above.

    My previous Cushcraft antenna, an MA6V, was mounted in the same position, and I had no detuning problems. Of course, it did not cover 40M. I do not have much experience with vertical antennas, so maybe the mounting location is my problem.

    I re-checked all dimensions and orientations of parts and traps, and they are per the instructions. To test the traps, I put one turn of wire around each trap and determined the dip frequency with my MFJ259. I found a site that listed the correct dips (with an MFJ) per Cushcraft, and I was close. Yes, it was a crude method, but I was trying to determine if the traps were working, and if the resonance was close to were it was suppose to be.

    As installed, the low SWR frequency on 30M is 10.126MHz (4.4) and on 40M is 7.25 (7.9)

    Originally, it appeared I had a bad 30M trap. But, I *think* the trap is fine.

    I cannot move the antenna to a better location now. After some ongoing yard work, I am putting a support in the back of the yard, near a 6 ft fence. The antenna would be 8ft off the ground, but near (5ft) from tree follage. That is the only other possible location in my yard.

    Any additional advice/thoughts welcome.

  6. #6

    Post

    Randy -

    I assume that this is a used R7, since this model has not been manufactured for over a decade.

    Did you open up the black mathcing box and inspect the inside ?
    Cushcraft's OEM supplier (Polycase) used steel screws into brass inserts ..
    SO use some WD40 or pentrating oil if these screws are rusted. Very easy to strip.etc.


    Most common problems with R-series matching boxes

    1. Static drain choke - shorts. Removing choke resolves issue - BUT no static drain.
    It should be replaced with a properly sized choke.

    2. Cracked toroid cores for the 4:1 matching balun (top) and/or choke balun (bottom)

    Bob at Balun Designs makes a replacement balun .. with better core material for 7 - 14 MHz coverage (R7 antenna).
    Contact him for details.
    ==
    Matching Box rebuild (with photos) - R5 and R7 are same matching box contents
    http://www.mrs.bt.co.uk/mrs/r5/

    Cushcraft R5 / R7 repairs
    http://pa0fri.home.xs4all.nl/Ant/R5/onhr5eng.htm

    MFJ will sell you a replacement matching box, last time priced > $ 160 USD.
    IF you price out new materials/labor that cost is appropriate with MFJ markups.

    w9gb
    Last edited by W9GB; 07-22-2011 at 03:11 PM.
    Nullius in verba

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