Antenna Changes

Now that I'm turning my attention back to QRP I decided that the antenna I've used for many years - an Inverted-L held up by (over the years) different supports, and currently a 10m SOTA-pole with the top few sections removed - tuned against a mess of radials and ground rods by a CG-3000 Auto-ATU - was due for a change.

The Auto-ATU needs “a few watts” to trigger a re-tune on a new frequency. Occasionally it'll work with 5W but not consistently, and certainly won't re-tune at anything lower than 5W. This means occasionally having to give it a carrier from a QRO radio to re-tune before switching to a QRP radio.

Not cool!

I'd considered a trapped inverted-L configuration, with traps for each band at the 1/4 wave points. But in a rare 60m SSB QSO with Stuart GM4WMM he told me he's using a multiband trapped End-Fed-Half-Wave. Fed via a 49:1 transformer, with the traps at the 1/2 wave points.

This removes the need for a good, low impedance ground connection and simplifies things. It also means “more wire” in the air for each band.

I removed the Auto-ATU and put my nanoVNA on the current antenna/ground feed point to see what I've got now (a totally random length). The first resonance/low SWR is around 3.2MHz. That means a 1/4 wave at 3.2MHz or around 23m. I can perhaps squeeze a little more wire into the space but I'll accept that the new antenna overall length will be limited to “around 23m”.

Next I wound a 49:1 transformer on a FT50-43 toroid (the largest type-43 toroid I could find), and added the usual 100pF capacitor across the 50 ohm input.

Connecting this to the antenna - and also adding a connection to the existing ground system, mostly for consistency, and some static drain etc, I found a very good resonant return loss dip at around 6MHz. And another at 12MHz and another at 18MHz. As it stands I could use it as a 3 x 1/2 wave on 17m….

My main band is 30m. So the next step, to allow me to use 30m while I build the traps, was to shorten it to 1/2 wave resonance on 10.1MHz.

I now have a resonant “End Fed Half Wave” for 30m.

No need for an ATU (auto or manual).

I made a trap for 10.1MHz and added it to the end of the current antenna, then added wire to make a dual-band 30m / 40m antenna. That should cover my normal CW operations for a while.

I found that adding the trap and additional 40m section caused the the 30m resonance to move up about 200kHz, so I had to add about 12“ of wire to the first part of the antenna to bring it back down into band. Otherwise it was a simple to add the trap, add the extra wire, trim the far end to bring the 40m resonance into band.

I'm thinking of also trying to get 60m, but a 1/2 wave on 60m is around 30m and won't fit in the space - UNLESS the loading effect of the 30m and 40m traps, below their resonance, will add enough inductance shorten the required wire length sufficiently.

I need to re-make the 49:1 transformer with a larger toroid. Some calculations would imply that a 50-size core isn't suitable for 5W. So far I've not seen any evidence of problems (I'm assuming overheating would lead to a loss of correct transformer action, and loss of the impedance transformation - and hence high SWR. This hasn't been seen yet on CW at 5W or on WSPR at 1W, so perhaps it'll survive long enough for me to replace it with a 140-size toroid, once I buy a few….