SWR on 2-m TV line

[Good4you]

I made and have been using a j-pole made out of 300-ohm tv ladder line. It works well for my application, and improves range from the rubber ducky. I mount this on a very small and tall fiberglass whip which i can get up about 20' w/o guys and maybe another 10' with string guys. iI hook this up to a 5w 2m handheld for a base camp communication for off-roading. I liked this antenna because it is very light and easy to get up high. I built a ground plane as well, but it is a bit heavy and clumsy to get up high enough.

I bought a SWR meter to try and dial the antenna in better. The problem is i get a SWR of just over 5 at the high and low ends of the band.
So my question is:
1) If the SWR is the same at both ends of the band, does that mean that my length is correct, but i have other issues causing the high SWR? (I built several of these from internet specs, and have about the same reading on all, even with minimal feedline.
2) What other factors could be causing high SWR? (I have tried wrapping some coax around the antenna base to no effect. My ground plane antenna gets 1.2-1.4 SWR. So the meter works.)
3) What would be a good antenna for my set-up?
4) Should i just give up on the J-pole and use a ground plane (this would mean an antenna height of maybe 5'-10' less due to weight.)

[5B4AJB]

Is the SWR at 145MHz OK?

If it is, you have a (nicely tuned) narrowband antenna.

There's not much you can do to broaden it by very much I'm afraid.

I had a mate who had one up very high and he used to be able to work stations others couldn't. He had tuned it for a specific frequency.

Just a thought... does your fibreglass whip have a wire inside (is it an antenna)?

[edit] Maybe the length of coax has some play too, try cutting it for a multiple of wavelengths.

[druid]

Instead of a ground-plane, how about a dipole? Maybe a bazooka? At 2M, they're only (about) 1M long, and don't need a ground-plane.

druid
VA7LAS

[mu0chn]

The first antenna I built was a Slim Jim, it was made out of 1/2" copper pipe with soldered elbow and as I remember the 2 vertical sections were about 2" or more apart. A J-pole is like a Slim-Jim with the 2nd vertical removed. It was quite wide band but that could be down to a few thing. The diameter of the elements, the gap between the elements. It may be worth trying to make a Slim-Jim with your TV ladder line just short the top like you do at the bottom.
How about some photos of it in use

[techiesteve]

Are you in the USA? If so I believe the 2M band covers 144 - 148 MHz, wider than our 144 - 146 MHz. If so you'll find it harder to adjust for a low SWR across the whole band. Your hand held is likely to be FM only, so why not tune it for a low SWR in the middle of the frequency range you want to work? Dem asked what the SWR was like at 145 MHZ, this was to check if there was something drastically wrong with your antenna setup, as it should hopefully be low in the centre of the band. An SWR of 5 is far to high to use. I have a slim jim made from 300 ohm ribbon cable, it shows a healthy low SWR at the FM part of our band as it was cut and adjusted for that limited frequency range.

[K7KBN]

When you say you bought "a SWR meter", did you get one rated for use on VHF? Most cheap CB-type "instruments" won't be very useful above 30 MHz.

[G6NHU]

I reckon so because he said the meter shows a good SWR with a different aerial on the same band.

[K7KBN]

It might SHOW a "good SWR" - but if the meter itself isn't designed for the frequency involved, I certainly wouldn't trust it. The J-Pole and the ground plane quite likely have very different impedance characteristics at the feed point. I've seen HF-only meters show a good match with a truly horrendous ACTUAL mismatch.

[Good4you]

the meter is made for 2m.
mid freq range is about the same swr (5).
im in the usa.
i primarily use a single frequency on simplex,so a very narrow bandwidth is fine.

I have noticed that most all antenna i have measured (including my mobile whip) shows similar SWR throughout the band. I thought this is how you are suppose to tune it. Does this just mean that all my antennae are close the proper length. I just have other issues on the high SWR antennae.

maybe i will try a dipole, it just seems awkward with the center feed-point.

[5B4AJB]

OK, so if the SWR is 5:1 throught the band, you have something wrong with the antenna or feedline.

Assuming the feedline is 50 Ohm Coax, not much can go wrong there (did you seal the antenna end of the coax to make it waterproof?).

Not sure how you can, if at all, adjust the feedpoint on a 300 Ohm ribbon antenna, try resoldering and re-cutting the gap???
Failing that, re-make the ribbon part of the antenna with the gap an inch lower or higher and see what the SWR is like on those...