2m j-pole on 70cm?

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Scott Moore:
There are basically 2 versions of the 2 meter j-pole. One is 1/2 wave and the other is 3/4 wave. Mine is of the 3/4 wave variety (as described in the ARRL Handbook). It's 58" long and uses a 19" matching stub.

I recently got curious and wondered how the same antenna would perform on 70cm. To my surprise, the SWR meter showed about 1.5-1. Now I am using a VHF meter - not necessarily rated for 70cm. Even still, I have tuned UHF antennas with this meter (2 homebrew) and they work just fine. Still, I don't know how much I can trust this meter on 70cm. I'm not exactly willing to be my final power transistor on it!

[As a side note, I have an MFJ-259B analyzer and have tuned this antenna for low SWR across the entire 2m band. Too bad the analyzer doesn't cover 70cm or my problem would be solved!]

The antenna is 2-1/4 wavelengths on 440 and the matching stub, at 19 inches, is about 3/4-wave on 440. I really don't know how to calculate this and even though I own a copy of EZ-NEC, I'm not very good with it and can't seem to model the j-pole on it with any success.

So what am I looking at here? I have read that a 2 meter 1/2-wave j-pole is resonant on 70cm but delivers a high (45 deg.) radiation angle and high impedence. However, this antenna is a 3/4-wave 2 meter antenna. What difference does that extra 1/4-wave on 2 meters make on 440? Is my antenna inherantly a true "dual-band" antenna that I can use with confidence on 70cm?

I'm using Radio Shack RG8-U coax, about 31 ft. (velocity factor of 78% already factored in). This calculates to about 4.8 wavelengths at 146 (therefore multiple 1/4-waves), and about 14.5 wavelengths at 440 (multiple 1/2 waves). If there is a high Z on 70cm at the antenna, perhaps it's being transformed to a low Z by the transmission line.

Your thoughts?  

Dan:
Let's start with 2m.

The J-pole you have, as are all J-poles that I've seen so far, is an end fed half wave on 2m.

The double part of the antenna (the bottom 19 inches) is a transmission line stub used to transform the very high impedance at the base of the 1/2 wave antenna to something closer to 50 ohms.  The line is tapped at a point that gives a good match.

- - - - - -

On 70cm, the 2m half wave ends up being 3/2 wave still with a high impedance at the base.  The 1/4 wave transmission line stub is now 3/4 wave, as you've pointed out, but it's still acting as a transformer.  Depending on where your tap point is, you might expect the high impedance of the 3/2 wave element being transformed down to something close to 50 ohms by the 3/4 section.  Just depends on where it's tapped.

- - - - - -

So, maybe you do have a somewhat low SWR on 440 MHz as well. The loss in the cable might flatten your SWR a bit; maybe the SWR at the antenna is more like 2:1 or something... but as far as your finals are concerned, you can transmit into that thing all day.  

Now, that doesn't necessarily mean it's a great antenna for 440... it *will*  have a higher angle of radiation than a 1/4 wave groundplane or 440 J-pole..

On the other hand, if you did happen upon a tap point that gives you a good match on 440, you're not going to be losing much power in the cable over the basic cable loss and the antenna will radiate.  So, in the absence of another 440 antenna, it might stand in acceptably.

I used a 2m J-pole as my main 2m antenna back when I was starting out too... I remember getting a low SWR on 440 as well.  I could hit a few close repeaters, but I did a lot better with a 440 1/2 wave over Jpole:

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|   1/2 wave
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----|  1/4 wave phasing stub
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|   1/2 wave
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| | 1/4 wave matching stub
| |
---

I have fond memories of hanging this antenna from my bedroom ceiling in Erie, PA and working all over Ontario into repeaters on 440 with 500mW when the tropospheric ducting fired up...

The collinear in my bedroom outperformed the 2m J-pole every time, and the 2m J was 15 feet higher and on a mast outside


73,
Dan
N3OX

Steve Katz:
I agree.  There isn't any such thing as a 2 meter "3/4 wave j-pole."  It's 3/4-wavelengths LONG, but it's still a 1/2-wavelength antenna.  The lower 1/4-wave is a transmission line transformer and doesn't count as part of the antenna's electrical length.

So, it's 3/2-wavelength on 70cm and that's a lousy performer.  It should load up fine, though.

WB2WIK/6

Scott Moore:
Good points, guys. Thanks for the feedback. I have a good 70cm antenna and a diplexer is on it's way in the mail so I can take full advantage of the dual-band capability of this radio.

Dan:
Good ... you'll be happier with it.

With a 6.3 inch 1/4 wavelength, there's no reason not to use a compromise antenna for 440.

I remember now I built a double version of that collinear Jpole too and put it outside for a while before I bought a diamond dual bander vertical.

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--x-|
--x-|
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----|
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Fed where the x's were, stood off from the mast about a foot by an arm that connected to the middle of the antenna.

All #12 wire, plastic coat hanger supporting the elements.

It was ugly but worked pretty well.

73,
Dan

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