Friday, January 25, 2013

HF Antenna: All Band Doublet

Another thing that I did over the Dec/Jan holidays was finally get an HF antenna back on the air, hanging from the south outrigger on my combination TV / Ham tower. I use a long rope to pull it up into position so that I can maintain it without climbing.

It is very simple, essentially a 40 meter band  dipole antenna (roughly 33 feet per side) fed with 300 ohm twinlead. This is commonly referred to as a doublet. At the bottom of the tower, the 300 ohm twinlead is connected to an LDG 4:1 balun, then 50 ohm RG-8X coax runs into the shack and is connected to a LDG AT-11 Antenna Tuner, then a NCG 2050 HF wattmeter and to my Yaesu 857D transceiver.

If you want a picture of what that looks like, check this out.

This is pretty much the simplest possible all band antenna you can make. It is only resonant at 40 meters, but the antenna tuner will "transform" the non 50 ohm impedance to the 50 ohm impedance required by the transceiver.

Some hams will argue that the antenna tuner is not perfect, and creates losses, which is absolutely correct (for more info see this reference). However, it is very handy from an operational perspective that you can use one simple antenna to work many HF bands.

My setup will actually tune and work from 80 meters to 10 meters. One ham commented on the air that you have to be careful using an 80 M dipole on 40 M, as that that creates high impedance, high voltage nodes at the center insulator. I find that it works for me at 100 W without any apparent issues. The transmission line is a random length, and is obviously transforming the impedance into something that the wide range LDG tuner can handle.

That is what I find so interesting about AC frequencies, especially at RF.... they act SO different than DC.

Have worked a number of contacts on 80M and 40M, and they are giving me good signal reports. Of course without another antenna to switch over and compare it to, "good" is pretty relative. When it gets warmer again I want to solder on some more wire to extend it to be a 80 M dipole and see how that works. I have tress to hold the ends pretty much as a flat top.

Also, I may putting in some switching arrangement so I can feed it as a "T" against my UFER ground (and maybe a couple of radials along the edged of the house at ground level) on 160 M. All you do here is tie the 2 halves of the 300 ohm twinlead together at the bottom and feed that with the center of the coax. The shield of the coax goes to ground (preferably lots of ground level or elevated radials, from what I have read).

You can read more about my UFER ground here.

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