Antenna building instructions / article for Radio ZS
This is about the easiest way to build a 160m antenna. It’s up to you to get it as high as possible. An inverted L shape is good, even vertical if you can but if you have no other choice get it off the ground if only a few meters high. You’ll still have some NVIS (Near Vertical Incidence Skywave) fun with really good signals – after dark!
Firstly, you’ll need a 100 meter length of copper wire that will be strong enough and good enough and the ideal for this is a 100m roll of 1.5mm electrical house wiring wire from your nearest electrical outlet. It will cost around R200.00 If you want everyone to see your wire, choose red or green / yellow but if not, choose black.
Unroll the wire making sure there are no kinks, loops or tangles. Take your time and enjoy the sunlight! Once unrolled, measure off 20 meters and cut the wire leaving hopefully a length of 80 meters behind. The 80 meter length of wire will be your radiator and the 20 meter length will be your “counterpoise”. You’ll need the counterpoise otherwise RF will be looking for a path to ground in all the places where it shouldn’t!
Good news, you’ll only need a short piece of coax from your rig to the magic box. Inside the magic box you’ll have a toroid with a few turns of wire around it. You’ll also need to find a variable capacitor which may be the most difficult obstacle. Find one that is around 200 pF or more. The vane spacing is usually not critical unless you want to use a few hundred Watts where you’ll need wider spacing otherwise you may encounter arcing between the plates.
We are going to feed the antenna from the side, not in the middle like a dipole or inverted vee. That means we need to match the rigs 50 ohms to the antenna which is 2750 ohms – or more. That’s easy of course using a low pass filter and it will supress some unwanted harmonics too!
My friend Pierre ZS6A, drew up an interesting table of suitable inductance values for various bands and feedpoint impedances.
From the table we see we need an inductance of around 32uH if the feedpoint impedance is around 2750 ohms. If it’s higher we’ll need a little more inductance but it’s impossible to know exactly what the feedpoint is. An inductance of 32 uH worked well for me and a 200 pF variable capacitor was sufficient too.
You will need a suitable toroid. I had a T157-2 in my parts box and used that. This is a red toroid suitable for HF frequencies. An excellent website on determining what toroids you have available in your box can be found at http://toroids.info/
There is also an online winding calculator which you can use to see how many turns you need to make on your toroid to get the required inductance.
There is an excellent program you can download from the Internet to your PC to make calculations on your toroid inductances. It can be downloaded from http://www.electronicecircuits.com/download/software/mini%20Ring-core%20calculator%20program.zip
This program gives you additional information like the affects of power on the toroid. For QRP 5 Watts, the voltage on the toroid would be V = SQRT (P x Z) and with a feedpoint impedance of say 3000 ohms, 5W would look like this. I’d loose 300 mW through the toroid which isn’t that bad at all!
At 100W the T157-2 would start to take a little strain as far as heating is concerned but that’s quite normal! An odd 7.4 Watts are lost through the toroid. Tradeoff …. lots of coax feeding the wire in the middle like in a dipole with the logistical complexities of a very large dipole or the end fed. The end fed offers many alternative options.
Note the 20 meter length of wire is connected to the earth side of the capacitor – make that the rotor then the control shaft is also at earth potential so there is no chance of getting a RF burn while tuning. Of course that’s the same potential of the rig via the coax braide.
Tune for minimum SWR. It’s pretty easy to get 1.1:1 with this setup but don’t leave the counterpoise unconnected! The counterpoise can simply lie on the ground.
Note, I built mine into an old modified MFJ945e antenna tuner with built in cross needle SWR meter. It can be much simpler, the toroid and capacitor in a suitable little plastic box.
At the far end (80 meters away) , terminate the wire on an electric fence insulator and tie down with a nylon rope. The ends are at a high voltage low current point.
Enjoy 160m, there are quite a few ham radio operators in ZS who would love to have a QSO with you!
73 de Eddie ZS6BNE