Author: webdev

At the meeting on 1/2/19 I described how we might use the club’s antenna analyser to best effect.

An important part of using the analyser is to understand the idea of standing waves.

I found this good illustration on youtube:

Hang in there, it’s a little slow moving but the illustration towards the end gives a really good visualisation of standing waves when there is a correct load and an open circuit.

How to tell if your SWR bridge is misleading you.
Have on hand an extension length of coax. It should be a ¼ wavelength long for the band you are experimenting with. After measuring your SWR, insert the extra length in the line – this moves your testing point 1/4 wave along the coax to a different point on the standing wave.

Here are some useful lengths:

6M band use a length of 1 metre, 2M band use 340 mm,70cm band use 13cm.

It’s not too critical. Now if your two SWR readings are similar (e.g. 1.2 and 1.3 then you probably don’t have a big problem but 1.2 and 2 would be unacceptable and require more investigation.

To actually calculate the length of the 1/4 wave section the formula is:
75*0.67/freq in MHz and the answer is in metres.

e.g. for 2M it’s 75*0.67/144.0 = 0.348 metres

I’ve used 0.67 as the velocity factor, this is true for many coax cables. Heliax is more likely to be 0.8.

John VK5DJ

Peter VK5BE and John VK5DJ visited the VK5RMG site on Monday 7th January 2019. This followed a visit the previous day to install the new link chassis and filters.

The link receiver from Naracoorte was program damaged (it lost its default frequency). A quick burst with the programmer at home and all is sweet. The whole system is now working with a somewhat faster ID. That’ll test your CW skills.

John (showing as Webdev)

What is FT8 and how do we use it ?

WSJTX is now in a new version 2.0 and is no longer compatible with previous versions.

If you are having trouble decoding signals it may be that you don’t have the latest version. Update and it’s like magic, back to copying people and working them again.

Here’s the link: https://physics.princeton.edu/pulsar/k1jt/wsjtx.html

After meeting President Peter at Penola on Friday 12/10/18 at 8:00AM to collect the antenna analyser and climbing harness, we refused to have him in the car as he appeared to be near death with the flu. We sent him home to bed.

Our working bee  began in earnest at Naracoorte to remove the backup antenna from the tower at VK5RNC. Tom VK5NFT climbed while John VK5DJ manoeurved the ropes and avoided a nose bleed caused by climbing higher than 1 metre.

Once at Willalooka we were joined by Alan VK5ZLT with his recently (that morning) finished 2* 5/8. A quick test with the club’s antenna analyser showed a nice broad banded response with SWR less than 1.2VSWR across more than 5MHz and relatively low reactance across the same spread.

So with the aid of his ladder Alan ascended to a lofty height of 6 metres to temporarily attach the antenna. The antenna replacement was necessary as we currently have intermittent problems with the main antenna losing signal strength at a distance and mixing interference from an AM station near Horsham. We suspect a poor connection at the antenna or damaged coax. The working bee was held to establish a test if the main antenna/coax is the cause of the problem.

Unfortunately an interference problem at one site has an effect on the whole four linked repeaters. We now need to monitor for a few weeks before tackling the next step.

Results as Tom and John travelled south indicate we haven’t lost much (if any) range as a result of the low antenna, although toward Kingston it may well be shaded by the tower.

What’s next? Once we know where the problem lies we’ll get a tower climber to replace the old antenna and install better coax than the existing RG213.