6
$\begingroup$

This was found among some old ham stuff and is believed to have been used on 2 meters.

The vertical elements are 17 1/2" long and the connector on the bottom is a standard SO-239.

Antenna (left) and Connector (right)

(See test results below pics.)

Antenna (left) and Connector (right)

Edited to add Test Results: I mounted the antenna to a small mounting bracket; just basically something to hold it but provided no ground plane. Then a ~20' length of RG-58 to an SWR meter and a 3' pigtail to my HT.

I got an SWR of 1.5:1 on 146.52Mhz and 1.2:1 on 446.000Mhz.

I also got a good, clear signal report on a repeater on 443.525Mhz that's about 11 miles away. The antenna was inside, leaning against my couch.

Edited again to add: There was a comment about impedance matching being done in the mount. There is not. Here is the mount disassembled.

Mount Exploded

This is the inner assembly with the outer shell removed: Outer Shell Removed

|improve this question
$\endgroup$
  • $\begingroup$ friendly reminder: please accept an answer or clarify what is not satisfactory about the answers you've gotten. This site stops working if askers don't give feedback!! $\endgroup$ – Marcus Müller Apr 15 '17 at 10:24
4
$\begingroup$

I'm sure it's a simple monopole for 2 metres. Mount it through a hole in a large metal sheet.

17.5" plus the distance from the bend to the groundplane, is almost exactly 1/4 wave on 2 metres.

Why two whips? I've never seen this before, but it will make it slightly broader band than a single whip. The effective radius of a pair of wires is (from memory) $r_{eff}= ({r_{wire}}^2+(spacing/2)^2)^{0.5}$. The wind resistance of the two wires might be lower than that of a single thicker conductor.

A quick note about 70 cm: often a 2 m antenna will also resonate at 70 cm. Being about three times the frequency, the monopole is 0.75 wavelengths long at 435 MHz. Unfortunately, the radiation pattern of a 0.75 wavelength monopole, or a 1.5 wavelength dipole, is far from ideal - it has a null at the horizon. So although the SWR is good, the effectiveness is poor. The longest you can make a monopole is about 5/8 wavelengths, which needs a coil at the base to add some inductance. With care you can make this coil work for both bands, but that's a new antenna design entirely.

In both cases the antenna depends on a reasonable ground plane, at least 18“ square, or a vehicle roof, etc.

|improve this answer
$\endgroup$
  • $\begingroup$ I don't know what else it could possibly be, other than a 2m monopole. Perhaps the ham who made this wanted to receive above and below the 2m band, and/or get the VSWR as low as possible. $\endgroup$ – Mike Waters♦ Mar 25 '17 at 21:44
  • $\begingroup$ Sorry for the newbie question: would you mind explaining why the 5/8 wavelength antenna needs a coil for inductance? How's the inductance help reception? $\endgroup$ – horse hair Mar 28 '17 at 3:28
  • $\begingroup$ Using two whips is like using a thicker conductor, so would be more wideband, I suppose. As for the question about the 5/8 inch whip, it is the means of ensuring you are feeding the base at a point of minimum voltage and maximum current. A phasor, essentially. Do some searching on antenna theory and you will see the current and voltage distributions along various lengths (fractions of wavelength) and it should become clear. Reception is helped mostly by simple resonance, and the phasor is there to help match the circuitry to the antenna better. $\endgroup$ – SDsolar Mar 28 '17 at 7:06
  • $\begingroup$ @tomnexus I would accept this answer except it misses one important point: based on my test results (added in the question) the antenna performed very well with no ground plane at all. Do you have any thoughts on this? $\endgroup$ – Lance Apr 9 '17 at 14:19
  • $\begingroup$ @lance It may work without a groundplane, with some luck it might work well. It forms a dipole, one half is the 17" wire, the other half is the long coax. Depending on the length of the coax, esp with longer coax, it may have a low SWR at your frequency. However, 1) the radiation pattern will be far from ideal, having many lobes, 2) the SWR will depend strongly on the slightest detail of the cable length and position, 3) the radio itself will be part of the antenna, possible mic RF feedback, even RF burns. If you have an SWR meter, try running your hand along the coax and watch it change. $\endgroup$ – tomnexus Apr 12 '17 at 3:24

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.