4
$\begingroup$

Clearly, building a Yagi tuned for 2 meters will likely have the best directional performance. Putting that aside, I'm trying to get information on what antenna type will have the best overall performance in the 2 meter band in an NFM mode.

I have a 5/25 watt 2 meter FM transceiver that I'm currently using. I've been experimenting with a ground plane antenna that I built with 4 radials. Just using it here in the office (rather than up high and outside) reception seems "ok". SWR is currently 3:1, but I'm not using it for TX at the moment. I'm also hesitant to try to tune it since I'm not sure if the ground plane radials are too long... or too short. :) I'm afraid that I could make things worse by trimming them.

I'm on a bit of a budget, so I'm currently building gear. In your experience (or if you can back it up with some theory), which type of antenna would provide the best overall (non-directional) performance? Building a dipole is definitely within my capabilities. I'm sure I can handle a j-pole too... but I'd rather not end up in an endless cycle of antenna building. :) I'd rather build the thing just once!

Update: Just for clarification, "working ok" means that I'm reading a signal off of a repeater that's 43 miles away in Hartford, CT with the antenna sitting on the floor of my office. Perhaps I just need to scale up, tune it and get it up above my roof.

$\endgroup$
  • $\begingroup$ Antenna building is fun! $\endgroup$ – Pete NU9W Aug 13 '15 at 16:19
  • $\begingroup$ The most important feature for the kind of antenna you are building is height above ground. Since VHF is mostly line-of-sight comms, the higher you are, the more distant the horizon and thus more distant communications are possible. $\endgroup$ – K7PEH Aug 14 '15 at 16:24
2
$\begingroup$

From the intended operating or design frequency point that you made the 3:1 swr reading go down 1 mhz and measure the swr. Did the swr go up or down? If the swr reading went down then the radials are too long and can be clipped 1/8" ata a time until <1.5:1 swr is accomplished.

If on the other hand when you change frequency down by 1 mhz the swr goes up, then go up 1 mhz from the intended operating or design frequency. If the swr goes down then the antenna is too short and you must add length.

K2PHD

$\endgroup$
  • $\begingroup$ Wow! Exactly what I was asking for. :). Took three years to get a straight answer. :) $\endgroup$ – David Hoelzer Jul 21 '18 at 1:36
4
$\begingroup$

Fundamental questions in a choice of antenna design are:

  • Does it have the proper polarization (vertical, horizontal, or circular)?
  • Does it have the proper radiation pattern?

Some secondary issues are:

  • Will the antenna be small enough for the space available, at the desired frequency?
  • How will it be mechanically supported, without interfering with the electrical properties?
  • Will nearby objects interfere with it, or assist it (e.g. car roof as ground plane)?
  • What type of matching network will it require?

The mode does not matter at all. The frequency only matters as it affects the size and therefore what is feasible.

You've said you want to do 2 meter FM, and that means that you will want vertical polarization. (This mainly means you don't want to build a horizontal dipole, which is a shame because it's really simple. Vertical dipoles work fine, but you need a horizontal structure to keep the feed line running away perpendicularly, which is why it's rarely done.)

This leaves you with a wide variety of vertical antenna designs — ground plane (with radials, or mounted on a metal roof), J-pole, bazooka dipole, coaxial collinear, et cetera. These can all work; it's largely a matter of what you want to build.

However, one factor we haven't yet addressed is the radiation pattern. You said “non-directional”, but there's no such thing as a truly omnidirectional (isotropic) antenna. Any so-called omnidirectional vertical antenna will in fact have a null pointing directly upward. In between directly up and horizontal, the pattern depends on the antenna design, as well as how high above the earth it is. What you want depends on the environment you're in and how stably actually-vertical your antenna is: if you have it mounted in a fixed location with an unobstructed horizontal view then you can benefit from using a higher-gain antenna (more horizontal, less vertical pattern).

Overall: Don't worry about it too much. Given that you're building your antenna, it's likely much more important to pick a design that you can build accurately and tune easily (because small variations in length can matter a lot for VHF).

$\endgroup$
  • $\begingroup$ This is extremely useful! That pretty much rules out the dipole completely, which is where I was heading. It's making me wonder if I should simply scale the ground plane that I've already built up to a full wave and mount it on a mast above the roof. I stayed away from the "omnidirectional" term and understand what you are saying. A toroid is just fine as far as I'm concerned, which is effectively what I'm getting with the vertical ground plane. $\endgroup$ – David Hoelzer Aug 13 '15 at 19:55
  • $\begingroup$ @DavidHoelzer I suggest you read some more theory. Ground plane antennas have to have a quarter-wave monopole element (the radials are a more complicated story); a half-wave or full-wave scale element would be an end-fed antenna (voltage peak at the feed point) which requires completely different matching. $\endgroup$ – Kevin Reid AG6YO Aug 13 '15 at 21:09
  • $\begingroup$ @DavidHoelzer Also, because radio signals are coherent radiation, making an antenna bigger than 1/2 wavelength in any shape at all (a ground plane antenna is 1/4 mirrored across the plane making 1/2) does not make it more efficient — but only makes it potentially have a flatter pattern (which, as already mentioned, does get you some useful gain). $\endgroup$ – Kevin Reid AG6YO Aug 13 '15 at 21:12
  • $\begingroup$ Super. I'm not so concerned with efficiency.. My primary interest is gain. $\endgroup$ – David Hoelzer Aug 13 '15 at 21:28
  • $\begingroup$ @DavidHoelzer Assuming the antenna is reasonably efficient (ie, you built it out of a good conductor like copper or aluminium, and not a poor conductor like carbon), the only way to increase gain is to make the antenna more directional. So "omnidirectional" and "high gain" are more or less mutually exclusive goals, in practice. $\endgroup$ – Phil Frost - W8II Aug 15 '15 at 13:37

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.