R7 Repair

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.

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

Traps usually go faulty because the heat shrink 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.

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!

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 bifilar turns (as against 11 in the pic above).

C - This antenna, being asymmetrically 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.

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 home brew R7 matchbox.. Not as pretty as the commercial one, but works just as well! )

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..

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 analyzer with a piece of insulated wire , configured as a single loop around the end of the coil on the trap assembly.

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.

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.

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 .