Modifying a Cushcraft R7 vertical ?

Discussion in 'Technical' started by K4TQF, Sep 12, 2014.

  1. K4TQF

    K4TQF Member

    I recently inherited an R7 from an SK. It has these short floppy radials around the base. From what little I have read about verticals & ground radials, I would think these are woefully inadequate. The reviews on-line run the gamut from "great performer" to "worst antenna I ever owned". I have dismantled it , cleaned the tubing, added some sort of no-ox grease for aluminum ( Permatex ) and re-assembled. Any opinions here concerning this antenna? or the use of longer wire radials?
    TNX, MD
    PS: the only performance difference I've noticed on receive, compared to a 20M dipole, is better on the bands above 20 M
  2. W5HRO

    W5HRO Administrator

    I had one of those ground mounted around 1987/88. The traps are really bad about coming loose and getting intermittent. You have to modify them with screws all the way though the tubing with a nut on the other side. Drill the holes through both sides. The stock screws work their way loose after wipping around in the wind for a few months. Make sure it makes good contact to the coil connections too. I think the screws or tubing made contact to the varnish coated wire where the bare part loops around the screws, but I cannot remember for certain.

    Other than that they work ok, but that was before I had the Desk KW though. No telling what the higher power AM would do to the traps, but I don't think they use caps if I remember correctly. I think they use tubing to generate the "C" so they might be ok for higher power AM.
  3. W5HRO

    W5HRO Administrator

    That's because dipoles have no gain especially above 40-meters. On the higher bands a Yagi or vertical with some gain will always perform better. Verticals usually work best for overseas stuff though whereas horizontal Yagis work best for the stuff here in the states on those higher bands. I bet why you think its working better on 20 is because your hearing stuff further away like overseas stations.
  4. K4TQF

    K4TQF Member

    You are correct, sir...

    I'll take another look at the traps... initially, I was only concerned with the tubing as this thing has to be 20+ years old...

    Thanks, MD
  5. W5HRO

    W5HRO Administrator

    You know, I just looked at the R7 manual and that's not the one I had. I had the older before the R9 that also covered 80-meters.

    I can't remember the model number for certain unless it was just the very first rev of the R9. The only thing I remember is that it was so tall it wipped around in the Oklahoma winter winds pretty bad and would lean over almost 45 degrees sometimes :lol:

    Anyway, the trap/loading coils kept becoming intermittent until I drilled the holes through both sides of the tubing and ran the screws all the way through with nuts on the other side. Your R7 design looks like it may be a little different.

    Update: Mine was the old AV-5 model and required full 1/4 wavelength radials. Totally different than what you have.
  6. W5HRO

    W5HRO Administrator

    You know I was thinking those newer R designs like the R9 could work for me on the top of my tower above my 10-meter Yagi. The only thing I don't like is the fact the ones that cover 80-meters are so tall that they bend in the wind really bad. That was the one thing I didn't like about my old AV-5 (5 bander) model years ago because it looked like shit even when I first put it up without the wind blowing. It was impossible to make everything straight up and down to where it looked uniform.

    Anyway, my old one didn't have the transformer matching unit and it required full length ground radials. That might be the issue though, that new transformer box at the base because of the greatly shortened ground radials. You might want to check to see how much AM power your R7 can actually handle. An R9 vertical on 80-meters here in California is really what I need to work the guys again back in the Midwest and on the East Coast, but I would probably melt down that transformer box with my HB transmitter the very first night. Chances are it would need to be modified to handle full AM power.

    Something to consider...
  7. K4TQF

    K4TQF Member

    Yeah, max power is not my issue at the moment... I'm more concerned with the dinky (factory) ground radials. From what I know about verticals, it seems to me that skimping on ground radials would be the last thing one would want.
    TNX, MD

    It's rated at 1800W PEP
  8. W5HRO

    W5HRO Administrator

    The matching transformer toroids in that box plastic box compensate for the short radials. I my case I would need to figure out exactly what they are doing and completely rebuild that box with heavier components and wire.

    Yeah, 1800 Watts PEP is like 636W RMS and then derate that by about 50% because of the over-inflated exaggerated bogus MFJ number and you get about 318W RMS.

    So, if you run no more than about 300W AM carrier power with 100% max modulation is would be fine. Going above that would result in a slow meltdown :lol:
  9. K4TQF

    K4TQF Member

    I'm too old school to ever trust coils in a box to do the work of "real" (counterpoise) radials... IMHO, that's just an RF "trick". I figured the "coils" in the box were for matching to coax... never thought about them compensating for short ground radials. But, so it goes with a "trap" vertical anyway... BTW, this one is 40M thru 10M.

    Most of the stuff I have read about verticals assumes ground mounting, with buried or on the ground radials... seems like 3 degrees is the magic number.

    300 watts sounds like a figure I would be comfortable with.

    I wonder if MFJ owned Cushcraft in 1993?

    Oh, well, it is what it is...

  10. W5HRO

    W5HRO Administrator

    Oh God...!!!

    Below are pics of the R7's matchbox. I think I take back what I said about the 300W :confused:

    First, you would need to shit can those caps and replace them with HV doorknobs then re-do the 4:1 balun by getting rid of what looks like black and white #18 wire. The last thing might be that dinky RF choke, maybe. The choke balun with the brown might be ok.

    The main thing are those caps and the crappy 4:1 balun with the greatly undersized wire. I'd get rid of the PC board with it's traces too and just hard wire and RTV everything inside of the box.

  11. K4TQF

    K4TQF Member

    IIRC, mine is more heavy-duty than the one in the photo you posted... or maybe not, now that I look at it.
    Don't know how that got turned sideways... a close look shows only about 6 months mfg difference in the two.

  12. W5HRO

    W5HRO Administrator

    Here's what the back of the board looks like from an old R5. This is the same unit the R7 has and this pic is very old.

    Anyway yikes... the thing looks really nasty. Last thing you want is a PC board with traces and pressed type rivet connections. This would all be really easy to fix though. New doorknob caps, new wire on both chokes, a heavier RF choke and hard wiring everything inside of the box along with some RTV. No more PC board and AM power meltdowns.

  13. W5HRO

    W5HRO Administrator

    It's too bad they don't make these antennas in dual or mono-band models. The only reason I would ever want a vertical would be for 75-meter and possibly 40-meter operation. The R9's just have too much excess garbage on them plus they are very expensive, $640 :confused:

    Anyway, I thought more about the modification of the matching unit and it would be better to just install a piece of perf board from radio shack in there just so you would have something to cable tie the chokes to and hold everything in place. Then you could still hard wire everything instead of relying on the crappy board traces. It dawned on me that RTV might soften and come loose it the chokes get warm or even hot.
  14. KM1H

    KM1H Guest

    Ive had to repair a lot of those R7 boxes for locals who fried them even with a SB-220. The top toroid, which is just a 4:1 voltage balun, the compensating and isolation/coupling caps that helped to broadband the windings, and PC traces are all a problem as was the RFC if it took a near lightning hit. I just wound a 30 uh air choke of heavier enamel wire; more than enough reactance. I rewired the 4:1 and the traces with #14 stranded Teflon that I bought in a 500' reel ages ago. When CC was just 15 miles up the road they would soak customers for parts or repair charges. Ive heard that MFJ cut that out.

    The R7 works well with real radials and nothing has to be changed or tuned. Most around here are roof or a short mast mounted as area ground conductivity is among the worst in the nation; elevated radials really wake that and other CC verticals up.

  15. K4TQF

    K4TQF Member

    TNX for the reply about the radials. I figure to stick this thing out in a clearing in the woods, and use guy wires as radials. It will be about 10 feet off the ground, just to clear the antlers on the deer... :lol:
    Soil conductivity around here is a joke. We only have about 12-18 inches of "soil". Mostly rotted leaves & roots !
  16. K4TQF

    K4TQF Member

    UH OH :surprised: Looks like the old R-7 is going to need some attention....


    Looks almost as bad on the upper bands... another summer project to add to the list :icon_thumbdown:

    Sorry about the crappy iPod photo... that's just shy of 5:1 @ 7.200MHZ.
  17. W5HRO

    W5HRO Administrator

    There was a page that no longer exists where a guy rebuilt the matchbox and I found one of the pics below, but the URL is gone now. What I would do is rewind the chokes like in the pic below and also replace the board itself with a piece of perf board then hard wire everything instead with heavy Teflon wire. Also replace that little undersized RF choke in the top left hand corner of the pic.


    Also below is the manual and some maintenance & repair tips for the whole antenna.

    Attached Files: