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I have backgrounds on electrical communications, but know little about EM/RF.

I would like to install outdoor antennas for experiments (receive only; I do not hold licenses, although I do intend to get one in the near future), and I want to use 4' - 8' marine antennas for this purpose. This is mainly because I want to place multiple antennas close together (so I can play with the phase differences of the signals), and yet have them tall enough so I can get line-of-sight to transmitters.

The problem is that I am trying to operate these narrowband antennas on frequencies that they are not designed to operate on. (Mutual coupling is an another problem, but I can forget about that for now.) I am not sure how well these antennas will perform if I operate them on higher frequencies, say, in the 900MHz range. What could I expect if I do this? Are there any alternatives, if I want to place multiple antennas close together?

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  • $\begingroup$ There are ways of electrically shortening antennas, like with a capacity hat. But anything like that would be a kludge. Have you really ruled out just getting an antenna designed for what you want to do? $\endgroup$ – SDsolar Oct 19 '17 at 2:25
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    $\begingroup$ A short antenna for a given frequency is usually inductive and the reason for adding a capacity hat is to counteract that inductance. So, using a capacity hat effectively lengthens an antenna. For example, I use to operate with a mobile center loaded antenna on my truck. It was a great antenna for 20 thru 10 but to achieve good efficiency on 40 an 80 I would install a capacity hat right above the center loaded inductor. $\endgroup$ – K7PEH Oct 19 '17 at 17:03
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    $\begingroup$ I agree K7PEH! It is used to make the electrical length LONGER, not shorter! So the problem gets even worse. $\endgroup$ – Keith Martineau Oct 20 '17 at 1:54
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If your goal is simply to gain height, why not mount multiple properly-sized antennas on a single mast? At 900 MHz, the antennas and a structure to hold them sufficiently far apart from each other would be quite small and light, so you don't need anything structurally complex. A prototype could be built out of wood, wire, cable-ties/screws/tape/glue, and whatever sort of 8' long object you can scrounge up.

And it will be unnecessarily hard to rig 8' long antennas to maintain specific phase (hence distance) relationships on the scale of inches!


The primary hazards of using the wrong size of receiving antenna, for casual listening, are:

  • It will be less efficient — this only matters if you are trying to receive weak signals, which I imagine you might care about if you're trying to locate transmitters at UHF frequencies.
  • You will pick up more low-frequency signals which could overload the input filtering of your receiver and appear as aliases or noise — the antenna's frequency selectivity is working against you.

In most cases of wrong-sized antennas for receiving, I would tell people that it's worth trying. In yours, it seems more potentially problematic and wasteful because of the large wavelength ratio and the specific application.

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  • $\begingroup$ Thank you! The efficiency is not a problem, but I guess I don't want to overload the receiver (without good preselectors) with low-frequency signals.. $\endgroup$ – piloteer Oct 19 '17 at 6:53
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    $\begingroup$ I completely agree Kevin. I believe Mike has a valid point about the pattern too. a LITTLE too big or small (compared to wavelength) is one thing. An antenna 4-8 times the FULL wavelength is another thing altogether. $\endgroup$ – Keith Martineau Oct 20 '17 at 1:51
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    $\begingroup$ Not to mention the radiation pattern goes up towards the sky as the antenna gets longer. $\endgroup$ – Phil Frost - W8II Oct 20 '17 at 15:40
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The problem with too-long UHF and microwave verticals is that they have an undesirable pattern. A vertical needs a very clean, low pattern with as few high-angle lobes as possible.

We want an antenna with a low-angle lobe that's directed as close as possible to the horizon (where signals come from!). 1/4 and up to 5/8 wavelength verticals will do that.

Much longer than 5/8, and you're mostly listening towards the sky, rather that near the horizon. And that's what you'll get at 900 MHz with those too-long vertical antennas.

You should also consider whether a good beam antenna with a rotor will fit your requirements. But with phased verticals, you can instantly change what direction you want to listen towards.

It sounds like you already have some marine antennas on hand. As Kevin pointed out, you'll likely have problems. Forget about using them for anything except the marine frequencies that they were deigned for.

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An 8 foot marine antenna is not a "whip" but rather it is a collinear array in a fiberglass radome. As such, it consists of 1/2 wavelength elements that are phased to provide approximately 6 dBi gain for the marine band (~156 MHz).

Operating any collinear antenna at a frequency other than what it has been designed for will likely produce very poor results. The individual elements will not be the correct electrical wavelength and the phasing of the elements will be wrong.

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