Tuesday, November 13, 2007

A Better Mouse trap?

I finally moved house and so the 6BTV could be re-assembled. Thankfully the gardens a bit longer at the new QTH but there’s no lawn and a garage filling most of the end. I guess we would all like to own a 4square on 80m but for me (And I reckon most hams) it’s about squeezing a quart into a pint pot.

One area of severe disappointment with the 6BTV for me was its performance on 80m. I know it’s a shortened vertical and the traps don’t help but that loading coil and whip at the top just didn’t impress me. I could hear lots of stations but even with 3 full size radials for 80M the signal reports I got were very low!

So I asked a few questions and came up with a new idea for 80m. Remove the 80M resonator and whip replacing them with a 40m trap and long wire.

Last month I finally bit the bullet and invested in an antenna analyzer (MFJ259B) I must say that having used it to set up the 6BTV I was impressed!

First I assembled the 6BTV without the resonator and tuned it for the higher bands (10/15/20/30M).

I had to install it a bit close to the garage which has an effect of slightly de-tuning the antenna but I was anxious to site it as far down the garden as practical. I did get a few questions from neighbours but I explained what it was and chatted about ham radio. The woman next door looked a bit dubious but I think I won her over? Or maybe she was just please not to hear any more about my pastime?

So now the challenge was to build the 40m trap. I read a few articles –


And downloaded a coax trap calculator -


So that’s all the theory. Now the practical bit…

I could have made the trap with a pvc pipe as former but I couldn’t work out how to mount it easily. I decided to throw caution to the wind and dissect the resonator! My idea here was to re-use it as the coil former for the trap.

At this point major surgery occurs. I am sure there are plenty of you who would like to know what is inside the 80m resonator so the following photos are an account of the disassembly of the resonator and construction of the trap.

First I removed the outer cover.

Next the four screws securing the top were removed, and unwind the coil.

Leave some of the coil to attach the ends of the trap to.

Remove the whip and wet the end with solder ready for the wire.

So this confirms what everyone has said. The resonator on top of the 6BTV is a simple coil. Remove the coil and construct the trap using “good quality” RG58 (Is that an oxymoron?).

The antenna analyzer was pressed into service to check the resonance of the trap (7.050 Mhz). The pickup loop was particularly useful for this job. Has anyone used the accessory coils they supply to use the analyzer as a dip meter?

I finished the construction with silicone sealant for the coax ends, a layer of amalgamating tape over the coil and plastic tap for weatherproofing. To ensure the tape stays put I used a black zip tie to secure it at the bottom.

The last job was to mount this on the antenna, Solder the wire to the top of the resonator and tune it for 40/80M.

I started with about 35 foot of wire and shorten it until I reached 3.59Mhz which is where we RTTY types live!

Its not as pretty as the straight 6BTV but it appears to radiate more efficiently (even next to the garage!). I managed to work most of Europe and about 6 US stations on 80M during the QTC contest last weekend. At the top end of the band the antenna doesn’t perform so well. So choose which part of the band you need.

I may add a trap for 3.8Mhz and add some wire for the RTTY end of the band? I was impressed at how easy it was to construct and adjust the traps!