Why antenna tuning unit is required for HF antennas and not for VUHF antennas?

Answer 1

First of all, all antennas used for transmitting should either be tuned by trimming or some other method such as an antenna coupler or tuner. It might even be a simple set screw that allows you to adjust the frequency up (by shortening the antenna) and down (by lengthening the antenna).

Still, there is an advantage is that the higher in frequency one goes - the broader band width the antenna. Plus, the difference in the wave length of higher frequencies is physically smaller. We will do some quick calculations in a minute for a HF band and a VHF band to show how that can be true.

A 19.5 inch ground plane 2 meter antenna will easily cover the entire 2 meter band with an SWR of 2:1 or below. Let's just take the band edges for example. (234/144=1.62 or about 19 5 inches) the upper end would be (234/145) or 19.3 inches. So in an ideal world 19.4 inches would be the center of the band and should cover the entire band. There is a whole megahertz of difference in that .2 inches. This does not consider the velocity factor of your antenna material.

Whereas in the HF band going from 1.8 mHz to 2.0 mHz (234/1.8=130 feet) (234/2=117 feet) can be as much as 13 feet in length difference. The difference in length is significant especially if you are going to transmit on the antenna. So on the 75 and 80 amateur radio band you might need several such antennas or an antenna coupler/tuner. There is only .2 mHz in those 13 feet which is less than the difference on you A.M. radio dial.

There are numerous other factors that effect antenna length and tuning. How far an antenna is above the ground, cable losses, and other far more esoteric factors. Of course, the larger antenna, the more the ground effects will be and the higher the frequency, the greater the cable losses... and on and on it goes.

Most TV antennas - since they are receive only don't have to worry about issues of VSWR or as more commonly known as SWR. They simply make a very broad band width antenna with different sized elements to help with making it not only more sensitive but aid in the band width. It would actually work better if you were to tune it to the channels you are watching, but since you are not protecting an expensive transmitter there is really not much advantage to doing so. It makes the antenna easier to install and more nearly dummy proof. Still, this may give some people the opinion that VHF and UHF antennas don't need to be tuned. That is a generalization that just is not true.

A quick thought about A.M. radio as we know it in the U.S. If we were to have ground plane antennas as in the examples above, we'd have some huge antennas. At 520 kHz, it would have to be up to 450 foot long, but would be too long for the upper end of the band at 1710 for the extend band requiring only 136 foot antenna. Wouldn't our cars look funny? Like long wired remote controlled cars. Fortunately, receiver antennas do not have to be the most effective antenna and there are other ways besides antenna length to tune an antenna. (Length remains the best way.)

The ARRL has several books on antennas including general ones, others on VHF and others on UHF and above. You will see in every example that the antennas are trimmed for the frequency or frequencies that they are used on at the location they are going to be used or an antenna tuner used with them.

For receiving - the longer - the higher - and the bigger around the more it will capture, but you'll notice a greatly improved signal if you tune your antenna for the frequency you are listening on even if it is several band wave lengths longer than needed.

So the question is, why are you not tuning your VHF and UHF antennas?

KD5KZL

Joe