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:
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:
| 13m Length at 28MHz | 11m Length at 50MHz | 3m length at 433Mhz | |||
| Calculated | Actual | Calculated | Actual | Calculated | Actual |
| 0.3 | 0.4 | 0.3 | 0.35 | 0.2 | 0.86 |
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 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:
| 13m Length at 433MHz | 11m Length at 433MHz | 3m length at 433Mhz | |||
| Calculated | Actual | Calculated | Actual | Calculated | Actual |
| 1.2 | 1.5 | 1.0 | 1.2 | 0.2 | 0.86 |
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…..
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