Thursday 30 July 2009

A 2m Slim Jim made from 300 Ohm ribbon cable

A project at my local club (Norfolk Amateur Radio Club) turned into a bit of a nightmare recently. The project was simple - let some of the youngsters make a 2m Slim Jim antenna out of 300 Ohm ribbon cable.

We had made plenty of these before and they work well. You cut a piece of white, translucent ribbon cable of about 60 inches long and bare the wires at both ends by about half an inch. You then twist the ends together and solder, giving you an overall length of 59 inches.

Then you cut a one-inch slot on one side about 18 inches up from the bottom. Feed it across the two elements about two inches from the bottom and there you go – except we didn't.

For this exercise we had bought some black, heavy duty 300 Ohm ribbon. When we made them up they came out on an MFJ analyser at about 136MHz – they were obviously too long.

After much messing around I decided that the only reason must be that the velocity factor of the black ribbon cable is different to the translucent stuff.

Out with a calculator and I reckoned it was about four inches too long. We decided to shorten one – carefully – until we got it right. We could then use it as a template for the rest.

We ended up with a length of 53 inches, with the slot 16 inches up from the bottom. This gave a virtual 1:1 – 1:1.5 SWR at 145MHz.

My prototype Slim Jim has now been put in a piece of white piping from the local hardware store with the silver top off a room spray on one end and the end off a toothpaste tube on the other.

It looks quite good and cost about £5 all in.

So if you are going to make a Slim Jim from ribbon cable do bear in mind that the length will depend on the type of cable you use.

HF Propagation Forecast June/July 2009

Here is the latest HF Propagation Podcast for June/July 2009 as featured on "This Week in Amateur Radio". Solar activity remains very low with virtually no visible sunposts. The Summer (in the Northern Hemisphere) is characterised by lower MUFs during the day than in winter, but higher MUFs in the evening/night. 14MHz may stay open later in the evening than it does in Winter. Sporadic E conditions are excellent, giving good short-range openings on 14, 21, 28 and 50MHz, occasionally getting up to 144MHz. Also available on iTunes

Friday 3 July 2009

The Hustler 5-BTV

I quite like the Hustler 5-BTV vertical. It is rugged, well made and gives you five bands in one antenna.

BUT - you must make sure you have a good earth radial system if ground mounted. Putting a single earth stake in will work, but it will be quite deaf. Adding radials makes a big difference. They don''t have to be resonant. As they are lying on the ground this detunes them anyway - just make sure that they are as long as the antenna is high.

In fact, there is no hard and fast rule about radial length, apart from more shorter ones are better than fewer longer ones.

The golden number to aim for is 120 radials, but that isn't really practical. As you add radials you will notice the SWR change and the performance improve, but as you add more the effect will be less - the law of diminishing returns. By the time you get to 48 -64 you probably won't see much difference.

In a test, I tried a 5BTV mounted on a single three foot ground stake - it was very, very deaf, although the SWR was fine. It was at least three S points down on a dipole and a Windom.

Adding just six 26ft radials made a big difference - it equalled the dipole and Windom and actually beat them at times.

A ground mounted vertical like a Hustler or Butternut is only half an antenna - put down a decent radial system and it will really improve.

Are coax-fed G5RV's lossy?

These were some tests I did with a G5RV mounted as an inverted V at about 25 feet and the ends at about six feet.

I first measured the coax losses of 30ft of RG58 with an MFJ 259 analyser, which were as follows:

3.6 MHz: (0.6db)

7.1 MHz (0.7db)

14 MHz (1.3db)

21.2 MHz (1.4db)

28.5MHz (1.8db)

I then took SWR readings on five bands - below are the SWR figures as measured at the bottom of the G5RV's ladder line with an MFJ 259 analyser. The figures in brackets are the SWR readings as measured in the shack after a run of 30ft of RG58.

Using the calculator at: the final figure is therefore the calculated total loss taking the measured loss and SWR loss into account.

3.5MHz: 2.0 (1.7) - 0.646dB

3.6MHz: 3.0 (2.2) – 0.724dB

3.8MHz: 4.2 (3.6) – 0.823dB

7.0MHz: 2.2(2.2) - 0.784dB

7.1MHz: 2.4 (2.3) – 0.806dB

14.150MHz: 4.5 (5.7) – 1.954dB

21MHz: 3.9 (9.6) – 2.83dB

28MHz: 3.6 (12.4) – 3.862dB

29MHz: 3.2 (9.5) – 3.397dB

As you can see the antenna was only really a good match at the bottom end of 80m and on 40m. The total loss gets worse as you go up in frequency. At 28MHz the combined loss was 3.8dB - you are losing half your power.

But the worst case scenario of 25ft+30ft of generic window line at an SWR of 12.4 on 28MHz is only 0.405dB.

So the conclusion is, a G5RV fed with RG58 coax is fine for 80m and 40m, but losses mount as you go higher in frequency. If you want to work the higher bands think about using open wire feeder all the way to the tuner. You then have a doublet with much lower losses.