Friday, 2 May 2008

Getting it up

The sealant cured on the slim jim, and its now mounted in its tube at about 15-20ft. Oddly, the SWR has gone up, and its now between 1.3 and 1.5:1 across the band, the lower figure towards the high end. Ive not had chance yet to do an on-air check, but receiving using the FT-290 plenty of repeaters and a fair bit of simplex is about. Interference from the ADSL modem was still bad, but some serious coil winding has removed most of it. I found the majority was coming from the network connection.

The HF doublet is shocking. Im blaming the change to a 1:1 coaxial choke balun, from the 4:1 collins. But the state of the feeder is also a worry, its been patched up several times and im sure there is still corroded sections. I have a 100ft roll of shiney new 450 ohm ladder line awaitin gthe rebuild. Probably i will go for a 135ft version, fed right back to the shack with 450 ohm. Im going to invest in a bottle of liquid tape for weather sealing the connections, and will also wind and solder the connections from the feeder to the antenna elements. Any splices in the feeder will be done to ensure continuity of impedence, and the feeder will be stood off from the fence etc along its path using electric fence insulators. I can then experiment back at the shack entry point with baluns etc.

I also took delivery of a 100m drum of 1.5mm2 conduit wire today, bought especially for antenna building, in particular for trying a 'stealth' 17m vertical mounted in the tree, with four tuned radials.

Thursday, 1 May 2008

VHF Omni

fed up of having no antenna for 2m, other than an indoor whip, and with the prospect of getting the beam up still a way off, i set about coming up with a cheap and dirty 2m omni. My plan was to build a coaxial collinear, but having taken stock of what materials i had laying about, decided that for simplicity i would build a ladder line slim jim.

Initially, i designed my own, taking account of the velocity factor of the line. This worked, but i couldnt get the SWR down below 1.5:1. The matching section was made from 1mm mains wire stripped, as 300 ohm ladder line is a begger to strip neatly.

However, a review of the designs had me adding a few centimeters more line, in accordance with other designs on the 'net. This new version comes down to 1.2:1 at 145.500MHz, rising only a little at the band edges. All this is at ground level and indoors. An outside test at 15ft with the original showed the SWR to come down.

Now, its pretty much useless as is, built on a length of wood. So a trip to the wholesalers brought in some 20mm PVC white conduit. The whole assembly fits neatly into a 2m length, with plenty of spare tube near the bottom for mounting the antenna. I did a 1min microwave absorption test on a sample of the tube, and it was barely warm, no more than would be expected from being near a cup of hot water. A few holes drilled allowed cable ties to secure the antenna inside the tube, and some sealent in either end waterproofs it. The seal is domed carefully by smoothing with wet fingers at the top, whereas the bottom seal around the coax was done by shoving the mastic nozzle right in and giving it a damn good squirt. Some self amalgamating tape finishes it off by sealing the cable tie holes. Its attached to my test 100ft RG-58 cable at present, eventually this will be severed at about 5ft and a BNC added.

All that remains, once the sealant has cured is to mount the antenna and test it on air.