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4m fm operation & antennas

Hi,All.
Would apreciate some advice,getting myself kitted out for the good wx we will be having soon?? Would like to try some activations using 4m fm and have ordered a woxun 4&2m handheld.Im going to try using a simple 4m wire dipole that Ive made on the basis it can be easily carried and taped verticle to a 5m fibreglass fishing pole.Do you think this will work ok or could I do better?Im an enthusiastic 4m fan and know it can be quiet out there but also rewards effort.Any suggestions?
Dave. G7CNM

In reply to G7CNM:

A lot of 4m activators use this J-pole http://www.summits.org.uk/tiki-index.php?page=4m+J-pole or something similar.

Regards,
Mike G4BLH

In reply to G4BLH:
Thanks Mike will have to make one and give it a go.
Regards,
Dave G7CNM.

In reply to G7CNM:

Yes, I have been using this j-pole for the past 18 months and find it very good … a bit better than a vertical dipole.

Good luck!

73,
Walt (G3NYY)

In reply to G7CNM:
Hi Dave

It’s great to hear that another Radio Amateur is keen to use this excellent band for SOTA.

The design for the slim jim is spot on if you use 300ohm ladder feed. When I made mine using 450ohm ladder feed I added an extra 30cm to the top section to make it work! In addition the fact that it rolls up and fits into a rucksack makes it ideal for SOTA applications.

I am pleased to say that at the time of writing I have used this Antenna, a 7 metre fishing pole and the Wouxun (single band 4m radio) on 137 activations and have had a plentiful amount of QSOs.

Have fun

73

Matt G8XYJ

In reply to G8XYJ:
Thanks Matt,Walt,
Hope to work you on 4m soon looks like wx lmproving at last!
All the best
73
Dave G7CNM

In reply to G7CNM:

I use a J-pole made out of 450 ohm ribbon cable. It can be coiled up and strapped to your rucksack. I fasten it to a 6m fiber pole with reusable tie wraps. It is easy to erect under almost any circumstances.

I made a similar antenna for 2m activation. To swap easily I have added ± 30cm section of coax to each antenna with an N-plug and use 5m low loss coax to connect to a 4m/2m handheld, me sitting under the fiber mast.

I have made many enjoyable 4m SOTA QSO in G, GW and EI land recently.

73, Phil ON4TA

In reply to G3NYY:

Yes, I have been using this j-pole for the past 18 months and find it
very good … a bit better than a vertical dipole.

Hi, Walt, isn’t a J-pole a half-wave dipole end fed with a matching section? I was looking at the dimensions to make one for six metres now we are getting some Es openings, but it looks simpler to make a dipole.

73

Brian G8ADD

In reply to G8ADD:

I would not expect much of a difference in the performace of a J-pole and a vertical dipole, but I think I can see why the former can be preferable for portable operation. If made from flexible material, it rolls up easily (as has already been said), and because the feedline is (mechanically) an extension of the aerial, it hangs straight when suspended from the top end. Not so easily done with a run-of-the-mill dipole.

In a pinch, you could also make a vertical dipole from an end of coaxial cable, just by pulling back a sufficient length of the braid. Not exactly waterproof, but any aerial is better than none at all :wink:

73, Jan-Martin

In reply to M0CEF:
.
Yes, the J-pole is only a couple of db better than the dipole, but very easy to construct covering two bands…if you use a mixture of twin lead and RG-174. I watched the inventor of the DBJ-2 make one last month, after which he gave it to me in exchange for 15 USD. I wind it into a 6 inch dia roll and squash that into a narrow oval suitable for slipping into a trouser pocket. Works for 2m and 70cm. Here’s his write-up:

Elliott, K6ILM

In reply to G8ADD:

Hi, Walt, isn’t a J-pole a half-wave dipole end fed with a matching
section? I was looking at the dimensions to make one for six metres
now we are getting some Es openings, but it looks simpler to make a
dipole.

Yes, it is. However, feeding a centre-fed vertical dipole with coax is problematic in a portable situation because if the coaxial feed line is allowed to drop down parallel to the lower half of the dipole it will unbalance the antenna and lead to a severe mismatch and loss of gain. Ideally, the feeder needs to run out at 90 degrees to the driven element for at least a half wavelength … not practicable if you are using a fishing pole to support the vertical radiator.

The J-pole, being an end-fed half wave, overcomes this problem quite neatly. You need to insert a balun at the feedpoint. Usually a coaxial choke balun is used, which is just a few turns of the coax feeder wound on a convenient plastic former, or just taped together.

Actually, the “Slim Jim” variant is what I use. It relates to the J-pole in the same way that a folded dipole relates to a regular dipole. If correctly matched, the Slim Jim has an extremely low angle of radiation. This has the effect of enhancing the apparent gain of the antenna compared to a simple dipole or 5/8 whip.

73,
Walt (G3NYY)

In reply to G3NYY:

If correctly matched, the Slim Jim has an extremely
low angle of radiation.

Interesting idea. Matching changes the angle of radiation? I don’t recall ever seeing that written before. What is the matching condition that gives this low angle of radiation?

73

Richard
G3CWI

In reply to G7CNM:
I use a slim jim for 2m activations using 300 Ohm ribbon. I have a 6m version using 450 Ohm twin feed high in a tree at home. Both work well.

73
Roger MW0IDX

In reply to G3CWI:

If it is not correctly matched, it will cease to operate as a Slim Jim and the radiation pattern will be unpredictable.

I should have thought that was self-evident.

73,
Walt (G3NYY)

In reply to G3NYY:

ROTFL. Thanks Walt. Priceless.

I wonder how my doublet works?

73

Richard
G3CWI

In reply to G3NYY:

Surely in the J-pole or Slim Jim, the feeder hangs down close to or even touching the matching section randomly as the wind takes it and is going to unbalance it making the performance unpredictable?

73

Brian G8ADD

In reply to G3CWI:

I wonder how my doublet works?

So do I.

:slight_smile:

73 de G3NYY

In reply to all,
your comments have been very helpful made me think of a few things I have not considered!
Many Thanks.
Dave G7CNM.

In reply to G7CNM:

In reply to all,
your comments have been very helpful made me think of a few things I
have not considered!

Looks like I’m going to need some 4m kit, Dave!

Thanks for the 2 calls today, fine contacts.

73 Mike
2E0YYY

In reply to G8XYJ:

"It’s great to hear that another Radio Amateur is keen to use this excellent band for SOTA. "

Not wanting to upset anyone… I understand the attraction of 6m and “below” and 2m and “above,” but I’ve always thought of 4m as having no redeeming features!

What exactly about 4m makes it “excellent”!???