11 meter dipole help???

[Zeb98]

Ok, I have made dipoles before but this one has me puzzled? I have been out of the radio business for about 10 or 12 years but I am jumping back in. I let my ham ticket lapse so I purchased a 11 meter rig on Ebay.

I made a flat top dipole using a chassie mount 239 connector soldered to stranded insulated wire bent back on itself and tie-wrapped on the ends for loop and adjustment. Using RG58/50 ohm coax. I tested and trimmed it about 8ft off the ground and got SWR to 1.3. I tied twine to a tree limb and the other to a point on the peek of my roof; about 14' off the ground...SWR jumps to 2.1? I adjust antenna to no avail; even tried a choke but just made things worse. I lowered the anchor to the base of my roof 9' and SWR 1.5? Keep in mind the twine hangs above the roof not the antenna for it is hanging obstructionless over my yard. And the higher I go the worse the swr gets but it's not stretching because the SWR is slightly lower on 40 than channel 1. If it were stretching I would assume that channel 1 would be slightly lowered? Unless the antenna is sagging slightly causing a difference is impedance...I am out of ideas. Any suggestions.

 

[wavrider]

Flat top or inverted V? Different configurations present different feed point impedance's.

1:1 current balun is recommended at the feed point, makes for a better signal and pretty much eliminates cmc, so when you are tuning the antenna you are really tuning the antenna not the feed line.

http://www.hamuniverse.com/dipivcal.html

good link with dipole calculator

welcome to the forum and good luck on the dipole

 

[TonyV225]

I also agree with adding a balun to that dipole. Had you tried an inverted V configuration?

 

[W5LZ]

Forget the balun, it's just not needed. And lengthen the thing maybe 4 - 6 inches on each side. Is that an absolute sure 'cure'? Nope, but I'll bet it lowers the SWR.
- 'Doc

 

[RickC.]

A balun would not likely have much effect on this, though it would eliminate what I suspect to be the problem.

That SO-239 for a feedpoint bothers me. It's not only bearing the weight of the feedline but is under stress when the dipole is hoisted up. Do you not have something you can use for a center insulator and wrap the coax around it and tie wrap it to hold it in place and bear the weight of the line, and solder the coax pigtails to the wire?

It sounds to me as though the wire and 239 flexing are causing your problem-- and/or possibly the PL-259 not being as solid as it appears and the coaxing crimping/flexing within the connector when the antenna is in a different position.


Rick

 

[wavrider]

Forget the balun, it's just not needed. And lengthen the thing maybe 4 - 6 inches on each side. Is that an absolute sure 'cure'? Nope, but I'll bet it lowers the SWR.
- 'Doc

Doc please enlighten me as to why a BALUN is not needed at the feed point?

 

[Superidgit]

Whats your length of each end of your dipole? or over all length?

Whats your feed point height?

Any way to make it an Inverted V?

 

[towerdog]

What sort of coax are you using. Any chance the coax runs parallel to the length of the dipole? I made a dipole for 2meters and had problem with the coax picking up RF. Simple fix was to have the coax feeding the dipole 90 degrees off. Also I hear you can use heavier coax, or run the coax through metal pipe to prevent it picking up RF. I got a dipole made from 7/8" heliax using the outer conductor, fed by double shielded RG-213 and got an SWR reading of 1.5:1 but havent had time to tune it up or play with it any more.

 

[RickC.]

I won't speak for Doc, but:

Would it be better for a dipole to have a well-designed balun? Well, yes.

Does it absolutely have to have a balun? No. I'd imagine most dipoles built over the decades have not had baluns, and that includes most of mine. I recall in the ARRL Novice study material from the 1970s (I helped teach a few classes then), that one of the disadvantages listed for baluns was , "It may not work"-- which I thought was pretty funny.

So, good to have, but not absolutely necessary-- and just based on the info we've been given here, I don't think the absence of a balun has any bearing on the SWR variations that are being seen.

Doc probably has a better explanation, but that's my two cents.


Rick

 

[wavrider]

Rick thanks for the reply.

In the old days most dipoles were constructed without baluns, heck in the older days it was single wire transmission lines

Today's antenna designs and modeling software allow one to see the actual distribution of current on a modeled antenna. Of course this is only as good as the software and the knowledge of the person doing the modeling.

Common mode current

About 2/3 of the way down this link it shows the dipole, no balun and the current distribution pattern.

Balanced antenna (dipole) fed with 1/2 wl or more of unbalanced transmission line (coax) the models show the coax radiating as much rf as the antenna itself.

So experimenters came up with this wonderful device, a BALUN, for connecting a BALanced antenna to an Unbalanced transmission line. Balun.

Place a 1:1 current balun at the fed point and the current on the coax is gone, the current distribution on the balanced antenna is back to where it should be. the antenna will now radiate a cleaner rf pattern as it is symmetrical again.

The dipole will work without the balun, the real question is how efficient will it be?

So yep it is not needed, neither is oil in a vehicles engine. The engine will still run without any oil, but for how long?

 

[RatsoW8]

Forget the balun, it's just not needed. And lengthen the thing maybe 4 - 6 inches on each side. Is that an absolute sure 'cure'? Nope, but I'll bet it lowers the SWR.
- 'Doc

Exactly. A balun is not going to fix the tuning problem and isn't needed with a resonant single band dipole.

If you can't get the swr close to flat you have issues with the wire, so-239, coax or mounting position. Trouble shoot until fixed.

I made a wire vertical for 11m last fall with an so-239 and insulated wire. Single vertical radiator and two ground radials hoisted up the inside of a tall ash tree. About 60' of RG-8X and the swr was no higher than 1.2:1 across all 40 channels with the radiator and one of the ground radial touching branches.

 

[Beetle]

I worked over 200 countries from 1959 to 1961, with 75 watts and a 40 meter dipole that worked well on 15 also. Don't know what the "SWR" was and didn't care; the transmitter loaded fine on both bands. Used about 55 feet of RG-58 and NO balun.

 

[Zeb98]

Thanks for all the advice. I have decided to rework the thing. The dipoles I made in the past all worked fine with low swr and I worked many countries on them but the difference is they were simply stripped coax directly soldered to the wires and secured without 239 connectors or fancy pvc housing.

I think I may have jumped into this project arse backwards; I soldered the wires in the housing first then started the trimming and testing on the ends. I figured 3" of line in the housing and probably had some difference in the two lengths internally possibly causing a slight mismatch. As to why the swr increases with height I still don't know; I did wrap the wire in and out of holes in the pvc to cause the pvc to take the strain and not the connector. RF does funny things. Anyhow a ground up rebuild is under way and I will measure and cut the antenna first this time then finnish up the soldering. I will let you know how it goes.:blush:

 

[W5LZ]

Most of the benefits of using a balun have been listed, but how about the disadvantages? As in it's more weight, it has to be a particular 'size' (have the right amount of reactance at the frequency used, they -are- frequency sensitive), is an additional cost, is an additional point of failure if not the 'right' balun.
And while a balun may help keep things symmetrical, what's around that antenna, how/where it's mounted will make things un-symmetrical to a larger extent than a balun can possibly 'cure'. The only time I have ever seen radiation 'slewing' make any difference at all was with directional antennas. Even in those times, that 'slewing' was compensated for quite easily, if it was recognized at all.
Baluns certainly do have uses, but they are not as important as commonly thought by most people. Think a balun may be of benefit to you? Use one. Then try the same thing without that balun, tell me how much difference there is. If you expect huge differences I'm afraid you'll be disappointed. If you do see large differences then you'd better start looking for other problems, cuz you got'em.
- 'Doc

 

[wavrider]

Most of the benefits of using a balun have been listed, but how about the disadvantages? As in it's more weight, it has to be a particular 'size' (have the right amount of reactance at the frequency used, they -are- frequency sensitive), is an additional cost, is an additional point of failure if not the 'right' balun.
And while a balun may help keep things symmetrical, what's around that antenna, how/where it's mounted will make things un-symmetrical to a larger extent than a balun can possibly 'cure'. The only time I have ever seen radiation 'slewing' make any difference at all was with directional antennas. Even in those times, that 'slewing' was compensated for quite easily, if it was recognized at all.
Baluns certainly do have uses, but they are not as important as commonly thought by most people. Think a balun may be of benefit to you? Use one. Then try the same thing without that balun, tell me how much difference there is. If you expect huge differences I'm afraid you'll be disappointed. If you do see large differences then you'd better start looking for other problems, cuz you got'em.
- 'Doc

Welll said DOC and thanks for the reply.

In today's crowded bands, and they are very crowded at times, the number one most useful purpose of a balun is to assist in preventing CMC, a cause of QRM. An rf choke will work but then there is that weight thing to consider.

They are frequency sensitive so having the correct balun for the antenna is a must.

I have built dipoles direct feed with coax. then put a 1:1 current balun at the feed point. The resonant frequency did change, several 100 hz than what the direct coax feed dipole was.

What made this freq shift? Balun or was the coax actually part of the antenna system?

I put a 50 ohm load on the balun and swept it with the analyzer, I saw very little reflected power with this test. This lead me to believe that a dipole, direct feed with coax is the TRI POLE antenna that has been written about in numerous articles, where the antenna is using the coax as a third element and the coax is radiating in the vertical field. Example is Beetles numerous DX contacts, verticals make great DX antennas.

Either way with or without a balun the antenna will work, it will make contacts.

If your QTH is in a crowded city, and cmc starts causing havoc on your neighbors electronic devices, computer speakers, then you may want to look at putting a balun at the feed point cuz you sure causing some other problems with that radiating coax antenna.