Antenna Length Assistance

[KC7NFF]

This Adam, I have decided on a 2m/70cm antenna and a 6m antenna for my mobile rig in my 99 wrangler. I really do not have any height restrictions as the wrangler is parked outside and it is a daily driver. Here in Coeur d' Alene, Idaho it is pretty mountainous and I was wondering what is a good height for these two antennas for best reception and ability to hit the repeaters. I am looking at 52 inches for 6 meters and 40, 57 or 62 inches for 2m/70cm. What would you guys recommend?

[AC0VH]

On 6m a 1/4λ is about 59", on 2m 1/4λ is about 19" and on 70cm 1/4λ is about 7".

The right length will depend on what you want to do as far as gain or not and if you have a good ground plane under the whip. Remember that gain is not creating energy but is analogous to focusing it. A 1/4λ monopole appears radiation wise to be mostly omnidirectional in all directions, similar to a dome with a little dimple in the end. But the antenna will need a ground plane under it to work.

If you increase the length to 1/2λ the antenna begins to focus energy along the horizon, if it's vertical. A true end-fed 1/2λ antenna doesn't really need a ground plane to work, but has a very high impedance and so to match it to your coax you need a matching network. Commercial antennas will have these built-in. A 5/8λ antenna will exhibit some of the traits of a 1/2λ but with a lower impedance such that they are easier to match to the coax. Not many HF and VHF antennas are longer than 5/8λ, sometimes you'll see UHF antenna longer than 1λ. Longer tends to increase gain up to a limit.

You probably should be looking for 1/4λ, 1/2λ or 5/8λ most commonly. If you are buying something this is all figured out, but since you are asking I assume you are building your own. If you are very concerned about deep canyons or valleys, the 1/4λ is the best option because there is essentially no direction over ground that is completely deaf and will be pretty much uniform in fact. In open terrain you give up some distance, roughly about 25%~30% technically. At 50W the difference might as an example 35 miles vs about 50 miles. But the 1/2λ antennas are not so directional that you'll probably ever have trouble in the mountains. I run a 1/2λ Larsen and it works fine here in the Rockies.

Point is that your 2m/70cm antenna is probably going to be no longer than about 38", which is 1/2λ on 144MHz. The commercial antennas of this length will put a coil in the middle that make the antenna look shorter at 445MHz, so that they are about 14" electrically, which is 1/2λ for 70cm or make them look like a collinear array (two antennas coupled together, the coil makes the whip look electrically like two antennas on UHF). This is to make them dual band capable. If you go longer than this you need to be careful to know why. There are legitimate reasons to build a 3/4λ or 7/8λ or 1.25λ antenna, but more often than not you will more screw up the radiation pattern for a mobile application. There is a lot of good reasons why most antennas sold by companies are around 19", 39" or 48" long for 2m (1/4, 1/2 or 5/8 of 2 meters).

On 6m if you can jump from 52 inches up to about 59 inches you can end up with an unloaded 1/4λ whip. With a good ground this will be a very good antenna. If you need to stick to 52" you might need some inductance to match it, but it shouldn't be much. I would definitely consider the right length whip since it's only a few inches more.

LMK if you have more questions. I would surf the Interwebs and get a sense of what is available commercially to get a better idea. Most mobile 6m and HF antennas are going to be heavily loaded with a coil to make them physically shorter, but 6m is right on the edge where you can get away with a full length whip and if you can this is much better than a coil loaded antenna.

[AD7AS]

Diamond and Comet both make a 6m/2m/70cm antenna that work pretty well. They come in two sizes ~40" which are shortened on 6M and ~60" which are full size on 6M. I would go with the longest one you can manage that will still be protected from trees when you go off road since they don't bend very much before the break/bend.