Friday, 27 September 2013

National Hamfest

Set off early for the National Hamfest this morning, should have been a nice quick run, but several accidents meant detours, one of which from off the A616, I had no idea where to go, but a local window installers van was in front of me, so I followed him! Arrived only about half an hour after opening, so wasnt too bad.

After paying in, which at a fiver I do think is a bit steep, especially since, despite this being our 'National' rally, its still not a patch on rallies of my early radio days such as Elvington and Drayton, I decided to do my usual 'tour' and take a look at everything before starting to shop. My plan soon went awry, when spotted beside a stall with a few PRC-320s on, was this

an almost complete, to CES, guying kit for the 27ft Mk1 Larkspur mast! Its missing a guy rope, the elusive spanner, and the 4lb hammer, but it does include the guy pegs ive so desperately been searching for, and the base insulator, plus a brand new, unissued, 1976 dated instruction plate! Now, ive seen these go for £40 on ebay without the pegs, so I set myself a £30 limit, and asked the stallholder how much "well, I was going to ask a fiver per item" says he, "or, £15 for the lot", well, I couldnt get a twenty quid note out fast enough!

But, it did give me one small problem - i'd been there under ten minutes and was already carrying a very heavy bag! So I obtained a pass out stamp on my hand and took it all to the car, before heading back in to start over again.

Although reasonably small still, it was none the less interesting, i'd have prefered to see more junk than computer stuff, but there wasnt too much computer kit. Some very nice masts which I can only admire, and even met one of our field engineers, who ive bossed about at daft o'clock, but never met. I managed to avoid the cup cake stall, although I was very tempted! and concentrated on getting what id gone for. Well, several 2nd hand N-types at 50p a pop filled that quota, 1N34 and 1N4148 diodes from Bowood, and the main items were accounted for. One stall had little sealed bags at a pound with a selection of trimmer capacitors in them, so some of those went in the bag. I took a bit of a punt on a 50p valve socket that may or may not have been the right one for the dekatrons, not as it turned out!

I also wanted some single sided PCB material as im running low. Most stalls with this were charging upwards of £2.40 for a 4" x 6" sheet. Whilst traversing the car boot, I spied some big sheets of it right at the back of a stall. These turned out to be double sided, but were something like 6" x 30" at least! I wasnt too keen at first, being double, as its a hassle taking one side of copper off, but when I asked the price and was told "20p a chunk", well you cant pass up a bargain like that! I bought five for a quid, and as you can see, it really was a bargain

My only 'frivolous' purchase was a little antenna, supposedly a dual 2m/70cm, according to the stall holder. Im not convinced, as its under a foot long, but he wanted a pound, and accepted the remains of my change, which was 90p! I'll need to test it somehow and find out what frequencies it works on. I also bought an antenna base spring for 50p, which might not be strong enough for the big HF whips, but im sure will hold a 10 or 12m antenna

Once home, via a well awaited Tower burger meal (large with Pepsi), I was very pleased to find my copy of Sprat awaiting me on the doormat. I could now turn my attention back to the 10m Wispy.

Firing it up, and looking at the spectra on the output of the mixer, I can clearly see that the fundamental, plus the third and fifth harmonics, are present along with the 28MHz signal, albeit at a somewhat reduced level. Setting the spectrum analyser up for observing the modulation, I can now see that the 28MHz signal is showing DSB, and the 3rd and 5th harmonics are much more AM like.

Curious as to whether im getting the maximum 28MHz signal, I looked again at my setup. Rogers original design stated 60pF trimmer, and I have a 35pF trimmer in parallel with a 33pF fixed cap. But, is it enough? The key to that would be if the trimmer was set at maximum. But these little ceramic ones are not easy to work that out with. So, taking my capacitance meter and the rest of the bag, I tested several, setting them to maximum and marking them. Maximum it turns out coincides with the top plates center being aligned with the single pin. Looking at my setup on the Wispy, it is indeed at maximum. I have a feeling that adding a bit more parallel capacitance will give me a bigger 28MHz peak, and allow me to gain that peak at a mid-range position. But thats a task now for another day.

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