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Copyright 1995 - 2008  - John TaitAll rights reserved.

R7 Vertical...Maintenance and Repair  

 

The Cushcraft R5 and R7 verticals are  favourites of mine, ever since  Alex EI6AG, my friend and "Elmer" for many years, gave me a present of a set of faulty R7 traps..   It was fun to repair the traps, and then "homebrew" the rest of the antenna, including the matching box.. 

Many thanks to Joe Reisert W1JR, whose advice helped me get it up and running. Thanks also to Dave Moore EI4BZ, who allowed me to dissect his R5 "Black Box" in the name of Science...!

I like the idea of the 1/2 w/l radiator, as against the usual 1/4 w/l fed against ground.. My "homebrew" version works extremely well, and I use it  as my "standby" and "reference" antenna. 

 

(Circuit diagram of the matchbox)

 

 

There are a few problems which arise from time to time in older units..  especially in wet and windy areas.. or close to the Sea.

   Varying, or high SWR on one or two bands.

This is usually caused by bad connections on the trap clamping arms. They should be carefully loosened, moved aside, and the contact area cleaned with steel wool. When clean, the area should be greased with Vaseline, or one of those dissimilar metal contact greases used by the electrical power utilities.

If this treatment doesn't work, and you still have high SWR one band, Then you have a ....

      Faulty trap.

Traps usually go faulty because the heatshrink seal on the trap fails, and water gets inside the coaxial "trombone" capacitors on the traps. If the inside of the tube gets wet, this changes the dielectric constant of the capacitor, and the value of the capacitor changes, thus moving the resonant frequency of the trap. You will find that the SWR minimum point has moved, and  normal adjustment will not bring it back on frequency.

  If this situation is not corrected, eventually the capacitor will short out, especially if you're running high power..

 Running high power into a bad SWR can eventually destroy the matching unit..!!!!    More on that later...

 

To repair the trap, you must carefully note the length that the capacitor "rod" extends out of the "Tube". The rod can slide in and out of the tube, and is one plate of the capacitor, so you need to get it back the way it was before you dismantled it...!

 

 

Carefully dismantle this capacitor. Dry and clean all of the components, and reassemble. Re-seal the tube with "self-amalgamating" tape.

All should now be well again... 

If the insulator tubing is badly damaged, you should go to Gerry VE6LB's page . He has a great method of repairing traps using Hot Melt Glue. When you see the original  damaged trap, you'll be amazed that it could ever be fixed.. Well done Gerry!

 

If you have high SWR on ALL bands, then you probably have a

 

       Faulty Matching Unit..

 

 

A - Matching capacitors.  These are a pair of 86pF in series, to give an actual value of 40/50pF. These can, and do go open-circuit. Replace them with a single HIGH CURRENT type (doorknob) of 50pF if you can. ( I use three 15pF caps in parallel)


B -
4:1 BALUN transformer. These can actually disintegrate with bad SWR and high power...Ouch !! Replace with an Amidon Iron Dust core T200-2 wound as in pic above.
   Or with a Q1 material Ferrite core ( FT240-61) wound with 6 bifiliar turns (as against 11 in the pic above).


C - T
his antenna, being asymetrically fed, needs an effective Choke Balun to keep RF  from the exterior of your feedline.
  This should not give any trouble...But if it does, an Iron Dust Core will NOT work here.. I use a ferrite core, Q1 material.

 

D - Make sure that all hardware connections are clean and secure


E -
Moisture release vent.


F -
Feed point (SO239)


G
PC board .


H -
RF choke effectively DC grounds the radiator to help prevent static electricity from entering your shack.
  If everything else looks OK in the unit, and the SWR is high on ALL bands..  Disconnect this choke. After heavy static, turns on the choke can become shorted. This damage may be impossible to see with the naked eye.

 

( Pic of my homebrew R7 matchbox.. Not as pretty as the commercial one, but works just as well! )

 

Take a look at the brilliant rebuild job that Darren G0WCW did on a wrecked R5 Match box. It's now working perfectly again..

 

Measure your trap frequencies..

It's a good idea to take a note of your trap's frequencies, in case you need to repair one in the future..

I measured my traps under the following conditions..

 
Disconnect Each trap unit from the rest of the antenna.
 
My measurements were taken with the trap unit resting on a large cardboard box, well away from all metallic objects..
 
I used my MFJ 259 analyser with a piece of insulated wire , configured as a single loop around the end of the coil on the trap assembly.
 
Vary the frequency on the MFJ until a strong dip is seen..
 
Now move the single turn link away from the end of the coil, while retuning for the dip, until a very shallow dip (least loading on the trap) is seen, and take note of the frequency read out.
 
Make sure that you do not touch any part of the trap while taking the reading.
 
These readings, though not strictly accurate because of the loading effect of the attached tubing and hardware, are very useful.
Make a note of your readings for each band, and keep them safe. If, in the future, you need to readjust a trap, then you can set it to the original reading.
 
I obtained the following readings from my R7...
 
       TV1  10m      26.5     mHz
                12m      23.2
       TV2  15m      20.05
       TV3  17m      17.5
       TV4  20m      12.85
       TV5  30m        9.73
 
You can see from these readings, how the hardware loads the traps, and gives a lower frequency reading than the actual.
 
Ken KA1VMR, who was very helpful to me, told me that Cushcraft, when making these traps, used a Signal generator and oscilloscope to set up the traps on the following frequencies..
 
       TV1  10m      28     mHz
                12m      24
       TV2  15m      21.2
       TV3  17m      18.11
       TV4  20m      14.47
       TV5  30m      10.25

 

 

 

Any manuals that you may require for these antennae are available in .PDF format from the Cushcraft website .

 

 

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Copyright 1997-2008 - John TaitAll rights reserved.

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