Diamond X-30N Capacitor Problem

Many radio hams have one of these white stick antennas. Mine is a Diamond X-30N, which is the smallest of the bunch, being 1/2 wave on 2 meters and 2x5/8 on 70cms. It is perched high on the roof and has given good performance.
I chose the Diamond X-30, because ham radio friend had an X-50, and that had given him trouble free service for many decades.
But mine began to give trouble. The VSWR became very high (over 3:1) , but only on the 2m band, 70cms seemed to be ok. Now it took me quite a while to convince myself that the antenna was at fault, because there was a faulty N-connector, but after several trips to the roof, I eventually decided that the antenna was not right. I think what swung it was realising that the 2m signal from the X-30 was nine "S" points down on a 1/4 wave vertical I had set up to compare it with. There was definitely a problem.

I set it up on a pole in the garden and found the VSWR to be rather variable, but generally around 5:1. Whereas the 70cms VSWR was about 1.2 to 1.

At the base of the antenna is a hex-socket grub screw, and if you loosen it, you can slide the internal gubbins out of the fibreglass "raydome".

Inside, everything was clean and shiny, with no sign of damage or water ingress. There is a coil at the base, with a small capacitor tapped on to it..

Now I was surprised how small the capacitor was, given that this aerial is supposed to handle 150 Watts. The capacitor is in series with the centre pin of the connector, so all the power has to go through the little cap. My suspicions were further aroused when I started reading on the internet that other people have had problems with this capacitor failing. The capacitor is marked 500V (it's Voltage rating) and 5C. 5 is the value in pF (i.e. 5pF) and C is the tolerance. The black tip to the capacitor means that it is an "NPO" type, which means it has a very low temperature coefficient.
Now 500V sounds like a lot, but you have to remember that is a d.c. value, and there is a significant derating to apply as you go up in frequency.

Note that some models of this antenna have two capacitors.

I took the capacitor off an measured it, and it actually measured 5pF, but I didn't trust it - ceramic dielectrics can be very peculiar things. I didn't like it - but what could I replace it with - there is no room for a transmitter type capacitor. I wondered about making a capacitor out of double-sided PCB material or coaxial cable ...
After looking around on the internet, I came across PA0FRI's excellent Website . It is hard to give a URL for an individual page, so you will have to follow his links to "antennas", and then to his modification of a diamond X-510, where he replaces the little ceramic capacitors with coax capacitors made from PTFE coax.
I remembered that I had some of that, so I decided to make one. The coax I used was RG178.

You can get the dimensions off the photo at left, but it is best to strip one end then trim it down to length using a capacitance meter. Remember to subtract the stray capacitance of the leads from the value that you read, and use very short leads on the meter, 5pF is almost nothing!

There is a complication in that I have removed the screen at the other end of the cable for several millimeters. This reduces the chance of a high-voltage breakdown at the end. I also put a blob of hot-melt glue on the end and covered it all over with heatshrink tube, so there was no chance of shorts to the coil in the antenna. But it makes trimming to length harder, and in fact I made a practice one first, to work out the length, then cut a "proper" one.

The photo shows the finished capacitor., attached to the coil. It folds around the insulator below the coil, and I found there was plenty of room for this in the tube.

Having put it all back together again, I took it outside and tested it in the garden. The VSWR was about 1.5 to 1, so not too bad. 70cms was also still good.

So I put the aerial back up on the roof. Checking the VSWR here gives a much lower figure of about 1.1 to 1 on 2m and about 1.2 to 1 on 70cms. So that is very promising.

I need to do some on-air testing, but in theory the PTFE dielectric should be very robust and stable. I will add a follow-up post when I have tested it more thoroughly.

73 Hugh M0WYE

Hmmm... maybe spoke too soon. The antenna worked well for a while, then went high VSWR again. Next morning it was low for about half an hour, then went high. Clearly an intermittent problem.

Having another look at the lower coil, I realised that the wire was not enamel coated, so uninsulated. Where the turns had been squeezed up, they were just touching. So I opened them out a bit. I also had a look under the blocks of foam  at either end of the middle coil. The middle coil is joined to the brass elements with a crimped brass sleeve. It looked like a good connection, but just to

be sure, I ran some solder over it.

The antenna is back up on the roof. Seems to be working well at the moment. Hopefully it will last this time. It has about a half an "S" point improvement over a 1/4 wave vertical in the same location. I think +3dB is about what you would expect for a 1/2 wave radiator.

73
Hugh M0WYE