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Scanner / Receiver Antennas For discussion of any type of receiving antenna used by a scanner or receiver base, mobile or handheld.

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Old 01-01-2011, 6:41 PM
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Default Which is the better antenna?

Which antenna, in everyone's opinion, would be better, the J-pole or the off center fed dipole? Pros and cons? Personal experiences? Thanks.
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Old 01-01-2011, 7:19 PM
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Originally Posted by sparks40 View Post
Which antenna, in everyone's opinion, would be better, the J-pole or the off center fed dipole? Pros and cons? Personal experiences? Thanks.
Uh - you might want to give us some more info - like what band???
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Old 01-01-2011, 8:12 PM
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You may want to check out the Wiki pages "under antennas", there's a bunch of the info your looking for on there, and it even has directions on how to build a J-pole I believe.
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Old 01-02-2011, 12:10 AM
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Strangely, although the J-pole is supposedly a narrowband antenna and the OCFD is supposed to be wide bandwidth, I've found that a J-pole made out of 300 ohm twinlead performs better than an off center fed dipole over a wide range of freqs.

Specifically, I've built twinlead J-poles and Slim Jim antennas cut for 124MHz (VHF airband) and 157MHz (marine band) and find they peform better than an OCFD. For the OCFD I used 18" and 48" elements.

I also found the J-pole significantly better than either a directly fed dipole or folded dipole fed with 4:1 balun. The dipole and folded dipoles were also built for 124MHz and 157MHz.

Theoretically, the dipoles should have had the same performance as the J-pole and Slim Jims, but the dipoles are noticeably less sensitive.

One thing that may be causing the abnormal results is that I'm using all of these antennas indoors, sometimes taped against a wall, sometimes hanging from a window sill. This probably lowers the impedances a bit.

The J-poles cut for 157MHz do a pretty good job in the 42MHz VHF low police band, and also in the 460-485MHz band. They are a bit less sensitive in the VHF airband than is the airband J-pole, but both are lightyears ahead of the rubby ducky antennas and even an outside mounted RadioShack 20-176 scanner antenna.

I don't have any test equipment, so my tests and trimming are based solely on comparing the reception on weak signals, preferably always on, constant strength signals such as VORs and ATIS for the VHF airband, and NOAA Wx radio just above the marine band.

It doesn't take much to build these simple antennas. Why don't you just build both the J-pole and OFCD and tell us the results.

======================

My next little mini-project will be a twinlead J-pole and a twinlead Slim Jim that are fed not by 50 ohm coax, but instead are fed by the same 300 ohm twinlead that I use for the matching stub. Then I'll use a 4 to 1 balun right at the scanner.

The only change in the design is a slight change in the total stub length, and movement of feedline connection point to about 2.5" up from the short. For 156.8MHz, and assuming 0.8 velocity factor for the stub and 0.95 for the dipole, the lengths I'll be using are 51" total; 35-3/4" for the dipole (or folded dipole for the Slim Jim) and 15-1/4" for the bottom stub. The 300 ohm twinlead feed will attach to the stub at about 2.5" above the short. (Calculations are 2" for the J-pole, 3" for the Slim Jim. I split the difference and try the antenna in both the J-pole/dipole and the Slim Jim/folded dipole modes. The only difference is whether the two wires at the top of the antenna are open or shorted together).

When I build the antennas, I will probably make the the feedline and the upper part of the tuning stub as one continous piece of twinlead, and then solder on the lower shorting stub where normally the feedline coax is soldered on.

N3GO http://snow.prohosting.com/~w0rcy/Jpole/jpole.html has an excellent analysis of the J-pole antenna and some useful basic and excel calculators.

Last edited by WA1ATA; 01-02-2011 at 12:23 AM..
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Old 01-06-2011, 4:45 AM
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OCFD - the main design goal is to be wideband. The cost is that at UHF and above, the strongest signals on those bands are from almost straight above, or nearly so. If you live in an extremely strong signal area, this may not be a big issue. Or perhaps you live in the city down low and get building reflections from above.

In other words, while the wideband matching may be ok, the directionality may not be. Below UHF, the antenna looks mostly out to the horizon like a vertical dipole.

It is also ok on Low-VHF too. But guess where most of the "antenna" is for the low band? In the coax-braid! The common-mode of the coax braid is doing most of the work here. For those not interested in VHF-LOW, it is highly recommended to CHOKE the ocfd at the feedpoint with ferrites.

What contributes to the coax braid becoming part of the antenna, is that the OCFD is inherently un-balanced being fed off to one side - AND the usual 4:1 TV type balun is a voltage-balun which doesn't do anything to help keep the currents balanced.

So, the OCFD is in my mind kind of a "fun" antenna to play with. If it works, keep it! Just don't take it to a hilltop or you'll be laughed out of town.

J-POLES - also great IF you choke them at the feedpoint. A lot of j-poles that aren't choked with ferrites, coax coils, etc are coupled in to the common-mode of the coax braid, which can skew the pattern. This may not be a big issue - it all depends if you REALLY want the antenna to do all the work.

Speaking of patterns, the usual VHF J-pole, will also work on UHF to a degree - BUT the directional pattern is now looking upwards on UHF like the OCFD! Maybe not to the same degree, but if a VHF j-pole is put into this dual-band kind of use, it is something to be aware of. Some may be better off with a purposely-designed j-pole for UHF that looks towards the horizon again.

OCFD - wideband, convenient, but look angles suffer at UHF and above.

J-POLE - will do dual-band, but upper band usually has poor look angle, unless designed not to. Needs choking near feedpoint.

Try them both!
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Old 01-06-2011, 9:12 AM
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Quote:
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Try them both!
Have YOU tried them both? Or are the comments above just repeating the common wisdom?

Last edited by WA1ATA; 01-06-2011 at 9:16 AM..
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Old 01-06-2011, 9:23 AM
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Originally Posted by WA1ATA View Post
My next little mini-project will be a twinlead J-pole and a twinlead Slim Jim that are fed not by 50 ohm coax, but instead are fed by the same 300 ohm twinlead that I use for the matching stub. Then I'll use a 4 to 1 balun right at the scanner.
The J-pole fed by a balanced line has essentially the same performance as the coax fed one, so it is the antenna receiving the wide range of freqs, not the feedline.

This also let me do a more direct comparison of J-pole vs offcenter fed dipole, since I used the same 4 to 1 balun with both antennas. The J-pole significantly outperforms the offcenter fed dipole, over a wide range of freqs.

I don't know what the antenna theory predicts as the comparative performance, but reality says the J-pole wins by a large margin.
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Old 01-07-2011, 3:16 AM
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Quote:
Originally Posted by WA1ATA View Post
Have YOU tried them both? Or are the comments above just repeating the common wisdom?
The OCFD goes back to when Bill Cheek and I used to build them together. That's back in the days of Usenet postings in rec.radio.alt.scanner in the early 90's. (Or if you were into logging into our networked landline bbs's on Fidonet). We both came to the same conclusion - fun, but not for serious use due to the common-mode interaction of the transmission line and high look-angles at UHF and above. BUT, in limited situations, this might be useful.

You might see me active in the original "why won't we let it die" OCFD thread here from years ago early on.

I've used J-poles as well, both made out of copper tubing and twinlead. Nothing magical, just end-feeding a half-wave. I only use them if I can't put up a vertical dipole fed in the center. Again, common-mode interaction is an issue for some if you don't choke it. Even MFJ puts a little ferrite choke near the feedpoint on their roll-ups. If current is traveling on the outside of a feedline, that may be a problem for some. It is for me.

The pat answer is that the j-pole is better. But we are comparing apples to oranges since the design goals of the ocfd and j-pole are different.

Last edited by hertzian; 01-07-2011 at 3:27 AM..
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Old 01-22-2011, 2:29 PM
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I built another variant of the twinlead J-pole that uses twinlead as the feedline. The twinlead feeds the scanner via a standard TV 4 to 1 balun. Performance is identical, to the limits of my ability to test, as the more conventional style of J-pole.

The all twinlead J-pole (or to be more precise, twinlead Slim Jim) is a bit more convenient to build and to temporarily mount.

The antenna and the feedline back to the scanner are one continous piece of twinlead.

Put about 1/4" notch about 1/2 wavelength down from the top, about 35-1/2" for 157MHz (VHF marine).

Solder together the wires at the top of the antenna. This also makes a good point for tieing a support cord.

At the point that would normally be the feedpoint, just strip off a bit of wire and connect a separate short matching stub.

For 157MHz I used dimensions of
top of antenna to 1/4" notch: 35.5"
Notch to point where shorting stub connects (feedpoint): 13-1/4"
shorting stub length: 2.5"

The 300 ohm feedline continues to where I have the scanner located. A balun with male F connector output goes directly on a BNC to F adaptor on my scanner.

Mechanically the antenna and the feedline are one continuous piece of twinlead, with the shorting stub hanging off of the antenna. This is a bit more convenient mechanically than the more normal method of having the antenna, the series stub, and shunt stub all in one piece, with the feedline soldered to the top of the shunt stub.

Last edited by WA1ATA; 01-22-2011 at 2:31 PM..
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Old 01-25-2011, 2:42 AM
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That's a very interesting variant of the J. Nice work!

One thing that has been on my to-do list is to try to broadband the J even further. I haven't had time to try it yet but here's the general idea - what do you think about this:

Start out with a 2M twinlead J-pole - the kind where the wire where you make the cut for the stub is left in place. In the past, I have snipped away the unused vertical wire, or left it in place - but this time leave it in place on purpose for:

Shorten the unused wire to create an "open sleeve" so that it might provide a little bit of broadening beyond 2M. Something like cutting enough to have about 2 inches away from the stub, and two inches below the tip of the opposing half-wave. There is no mechanical connection, it is just a half-wave for 150 mhz just sitting next to your 2m radiator.

I'm wondering if the spacing is too close for it to react like an open-sleeve and not matter much. Surely at some point the match will not be optimum, but I always wondered if this would extend the bandwidth upwards a little bit for forays into 150+ mhz...
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