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Trap Dipoles Ageeeeeeen….Part 3

[edited for links and notes, 2023/07/15]  The original trap dipoles were constructed using coils and caps.  Using Rg-58 for the coax style trap dipoles was rejected because of the weight and size of rg-58 coax traps.  Using RG-58 defeated the primary goal of making the antenna as light weight as possible.

Somewhere I picked up the notion of using rg-174 or rg-316 type mini coax to make the traps.  It looks like the voltage ratings on the rg-174 is higher(1100v rms), so that was chosen for the first experiments.  If luck holds out, the tiny coax will be sufficient for use on the dipole traps for a full 100w CW.  Using the smaller lighter mini coax will allow for lightweight construction from easily available materials that can be easily supported using telescoping fiberglass masts like those available from Spiderbeam, MFJ, or Jackite.  i.e., perfect for portable, field day, rover QSO parties, or POTA/SOTA.

The trap calculator program hosted by KC1KCC gave me some starting numbers to work with, and actual trap measurements came out quite close to the calculated values. [alternate calculator at K7MEM]   The traps are built with the coax coils wound reasonably tight to the form, and the coils were taped down with electrical tape prior to taking measurements.  These are all wound on small sections of the same sort of plumbing drain tailpieces that are 1.5″ od (38mm od).  (e.g., in the US available from Lowes or any hardware store selling plumbing supplies.)  The table below are of traps as built and tested with the nanovna.

frequency turns rg-174
27.7 3.33
20.66 4.33
13.75 6.1
6.75 10.3 [calculated]

Update, 2024-04-04

Received a new 1 inch o.d.(~25mm) form material that is lighter. testing.
Freq———–# turns calculated——#turns actual——-
27.7Mhz—>5 turns (approx)—–> 5 turns, 24.8Mhz(use 4.5?)
20.66Mhz–> 6.25 turns————> 6.25 turns, 19.65Mhz(use 6)
13.75Mhz—>  8.75 turns  ——–> 9 turns, 13.5Mhz & 13.7Mhz
6.75Mhz—-> 16.33 turns
END 2024-04-04 Update

Test Antennas:

The test antennas were built for the CW segments of each band. With the best SWR centered on the xx.070 area it will probably give enough coverage for both CW and SSB operation without a tuner.  An 80m/40m version will require tails for 80m adjustments.

Testing of two antennas was done before the May 2022 CQ WPX CW contest.  The 20m/15m version tuned easily….after I figured out I was working on that instead of the 10m antenna.  Read those labels, because at least I had them labeled properly when they were built several weeks earlier.  The 10m/15m version also tuned easily.

[aside: the 15m/20m trap is now in service as the skimmer station antenna, after a recent storm broke the 160m inv-l]

Although I missed the WPX contest, I soon got a chance to do antenna testing at 100w levels.   Both antennas handled the power easily with no signs of SWR rise.  Both were tested at 10 seconds, 30 seconds, 60 seconds  and five minutes of CW key-down.

Hoping for good conditions in FD to allow testing of the 10m/15m version.  Sunspots, do your thing!

[2023-07-15 additional notes]  The coax traps began showing swr problems on 10m after a few months in the weather.  Expecting this to be a problem with water intrusion.  testing the use of WeldBond glue as a sealant.  [alternative….Elmers ProBond]  Also testing the adhesive as a sort of q-dope to seal the coils on the form.

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