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I recently inherited a CelWave fiberglass antenna pictured below.

It is marked that it's for 959.9875 MHz.

The model number appears to be either 339 or 359. It is 96 1/2 inches from base to tip.

I checked it on my MFJ Antenna Analyzer and found it start to get a decent SWR over 460 MHz. I expected as much since half of 960 is 480.

Is there any way this antenna can be disassembled and retuned for use in the 70cm ham band?

I don't mind even if there is risk involved. It didn't cost me anything and I have no use for it as it is.

Antenna

Catalog Number

Measurement

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  • 1
    A model number and the dimensions of the antenna would be helpful. Since you have an antenna analyzer, reporting the measured complex impedance (with no coax in the mix) at your target frequency would also be helpful. – Glenn W9IQ Oct 8 at 11:02
  • Interesting that the SWR is already decent at half the frequency — usually a "1/4 wave monopole" becomes a "1/2 wave endfed" going in that direction, and that's NOT a good match situation. I wonder if that antenna was already working on a harmonic of its fundamental resonance? – natevw - AF7TB Oct 8 at 17:40
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    Without more detail on the antenna, there's not much to say here. Any antenna can be disassembled and modified. Will it be easy? Will it maintain desirable properties such as low-angle gain? No idea. To answer that would require a model number ideally, or at least a picture of the actual antenna with sufficient detail to reveal some information about its construction. – Phil Frost - W8II Oct 8 at 18:38
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    There's no reason an antenna should start working at Half the design frequency... generally the impedance might look good at 3 x the design frequency. This says nothing for the patterns though. If you can take the antenna apart, you could re-use the housing and connector, and build in a new 70cm antenna. – tomnexus Oct 9 at 7:04
  • I've edited the OP to add additional information. – Lance Oct 10 at 15:20
up vote 4 down vote accepted

The wavelength at 959.9875 MHz is approximately one foot. So this antenna is approximately eight wavelengths long. That means it is not a simple monopole or dipole. Based on the dimensions I would guess it is a co-linear array of dipoles with the objective of increasing gain towards the horizon by suppressing radiation at less useful high angles, while maintaining an omnidirectional pattern.

That the antenna has a decent SWR does not necessarily mean it's any good on 480 MHz. For a co-linear array to work as intended, each element must be in phase so they interfere constructively at the horizon. The phase of each element is controlled by transmission lines or reactive elements within the antenna, all of which have frequency-dependent behavior. It is unlikely the phasing components continue to perform their function at any frequency but the one for which it was designed.

Consequently, the antenna elements will most likely be out of phase, resulting in a radiation pattern which is worse than something much simpler.

A simple A/B test using a simple 1/4 wave monopole (or whatever else you have lying around, or can quickly improvise) as a reference should be sufficient to make a determination. If the mystery antenna isn't significantly better than the reference antenna, you can reasonably infer the phasing is all messed up and this antenna isn't going to be useful, even if it is a good match.

You certainly could disassemble it and modify it to work better, however that will be a more advanced effort. Even that may not work so well: halving the frequency requires doubling the size for an equivalent design. Generally it's easier to make things shorter, while making them longer is...harder.

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If the antenna is used at a very different frequency than what it was designed for, it will look either too inductive or too capacitive and it will have a different resistance. You can match it with a matching network like a pi or an L to get it back to 50 ohms. However, it will have low gain - but at least it won't kill your transmitter.

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    Your low gain assertion is questionable if you can match the antenna at the feed point with reasonable efficiency. The directive gain difference between a half wavelength dipole and an infinitesimal dipole is only 0.39 dB. – Glenn W9IQ Oct 8 at 11:13
  • Perhaps, though if the antenna is resonant on a longer wavelength than the one designed, that suggests the antenna is not simply a 1/2 wave dipole. Perhaps it's a co-linear array. – Phil Frost - W8II Oct 8 at 18:26
  • but surely there has to be a tradeoff.. otherwise you could use a 4cm whip to receive commercial band AM, provided you matched it to your circuit and expect it to have the same properties as a full wave antenna? your matching network helps to get your voltage and current back in phase. but it does nothing to alter the resonant frequency of the antenna. – Ryan Oct 8 at 19:29
  • The tradeoff is only in the efficiency of the matching circuit and the efficiency of the antenna. The directivity of a dipole hardly changes as you reduce its size compared to 1/2 wavelength. This is a commonly misunderstood principle. – Glenn W9IQ Oct 8 at 22:44
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    The gain of an antenna is directivity * efficiency. Efficiency is determined by the real part of the radiation impedance and the resistive losses. The real part of the radiation resistance does not drop significantly until it is a very short antenna compared to 1/2 wavelength. – Glenn W9IQ Oct 9 at 0:16

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