I am sorting out my gear and have two old amplifiers available free to a good home, provided you collect from north London. Both are for 144MHz and are for single 4CX250s. There are two sets of PSU parts, a changeover relay and one blower. One was given to me by a club mate, so is of amateur construction. The other I bought for parts at a rally years ago and is nicely made in a blue case. I was told it was made as a project by apprentices at Marconi, but have no way to verify that. If anyone needs a used valve or two for the project I can raid my box - but you can't have them all. Please contact me by email to arrange collection from North London. Here are some pictures taken in my loft (where the amps have been for almost 6 years - hence the dust!) using my iphone, so excuse the poor picture quality:

I have tried a number of multiband wire antennas at GB2GP over the last year and have never been really happy with the results. The current antenna is a 1/3 2/3 split OCF dipole, which seems to work but still causes me a few problems. The main issue is that there is huge BCB breakthrough from Brookmans Park which is in LOS, so I am using a high pass filter at the transceiver to reduce this to a reasonable level. The filter is designed for 50 ohm impedance and the OCF is no where near that on all bands and even though the radio ATU can tune out any mismatch we toasted one of the filter torroids during JOTA due to the mismatch.

The answer seems to be to use a resonant antenna for each band of interest. One of my neighbors, G0RPM, has had good results with a parallel dipole made by Alpha Delta. I called the UK importer Martin Lynch to enquire about these and was told by an unhelpful person they were out of stock and didn’t know when they would get any more in. As a result I have decided to stop being lazy and build one!

The plan is to have an antenna for the 80-10m contest bands. This means four dipoles, 80m, 40m (which also works on 15m), 20m and 10m which I measured out in my hall from wire I had knocking about in my store.

The next job was to build some spacers for the elements. A search on google suggests the spacing should be wide as possible to make tuning easier. I have a miniVNA which should simplify tuning so I decided to use only 75mm between wires. I picked up a length of PVC pipe from my local hardware shop cut it into pieces then 3mm drilled holes in my workshop (aka the kitchen).

Feeding the wires through the holes was a simple job, but securing the spacers was more difficult. I tried hot melt glue, which was partially successful, but ended up using cable ties to produce my ladder effect. I had a spare commercial 1:1 balun which I put at the feed point and the antenna is ready to go in the air. Hopefully, installation will be an easy job, but I hope that I don‘t get too cold trying to tune the thing as the temperature here is not predicted to rise above freezing for a few days.

I usually use either a fist microphone or a headset when on the air, but my recent experiences at GB2GP have shown me that a desk microphone is useful when operating a special event station as it is easier for visitors to use on the air.

GB2GP has two radios that don’t have microphones at the moment so I looked into building a couple of electret gooseneck desk mics from the ARRL handbook. Once I had costed up some parts that weren’t in my junk box this project felt too expensive until I saw PC desk mics on ebay for only £2 each. At that price I bought a couple and they arrived this morning. The external quality looked just fine - so far so good

Like any good radio amateur, the first thing I did was take one apart to see how I could add a PTT. As expected the quality inside was only worth £2, but the thing that really surprised me was the bag of grit that had been inserted to add weight to the base!

I guess it works, so why not stick with it, but I will be sealing the bag as I spilt half the contents on the floor when I tried to take it out of the microphone!

Next job is to change the flex to some 4 core and add an 8 pin microphone so they can be used with the TS-711 and TS-690S at GB2GP which shouldn’t take long.

At a loss

I haven’t blogged for a while – mainly because I have been too busy building stuff to write anything down.

The main focus of my activity has been rebuilding kit to install at Gilwell Park, the Scouts Association’s HQ which holds the special call sign GB2GP. I will write about antennas another day, but I have been rather stumped by some recent measurements I have made on co-ax runs.

The main coax runs at GB2GP will be Andrew Heliax LDF5-50 which is great news. 5-50 is a rigid cable so you have to use a loop of flexible cable around the rotator and other joints. As a result I bought some Ecoflex 10 from Terry at Diode Communications to do that job. It isn’t cheap, but has a good price:loss ratio!

Terry did a great job in rushing the cable to me in bad weather at his end and I made up three lengths to go from the antenna feed point, down the boom (where required), round the rotator and then to the main 5-50 run. These lengths were 13m, 11m and 2m. The antennas are being fitted on a commercial tower by professional riggers so I wanted to make sure the cable assemblies were perfect as there is no second chance to fix mistakes. Using mini db calculator I estimated the losses on the highest frequency that it would be used on and compared them to the losses measured using both my mini VNA and calculated from power read using a Bird 43 wattmeter. The table below shows the losses for each bit of cable (db) and one major issue:

0.86db loss from a 3m length of cable! Shocking! This was the only cable that used a PL259,so I immediately suspected this to be the problem. I called Henry Westlake and ordered some pressure fit PL259s to ensure that the braid was properly installed, but this made no difference at all.

I then wondered if the co-ax itself was not to spec and tested all of the runs on 432MHz as this could highlight any problems:

I am prepared to accept 0.2db variations from spec I had to use some inter series adapters and I am not using calibrated measurement gear, but it looks like I have a problem with that short run. If the PL259 is good, then the N-type at the other end must be the problem! What else can it be? I am at a loss…..