Choke or not?

[Stingray]

should i wind an rf choke just under my base antenna upon installation or should i check the swr first and see if it is ok?
and if it is ok without a choke should i do one anyway? Thanks.

[Circuit Breaker]

A choke won't do much for SWR. Chokes are used mostly for RFI issues. In some cases, a BALUN is needed to bring the SWR to a reasonable level...but check your SWR first. A BALUN shouldn't be needed.

[Stingray]

was just wondering because i wound a choke on my 102 that im using as a base antenna right now and the choke brought the swr down from 1.5 to almost flat.
(is it just fooling my meter?)

[Red Warrior]

A Balun and Choke can be the same thing. Sometimes they are actually called a choke balun (do a search on Google). The purpose of a choke balun is to prevent the shield of the coax from becoming a part of the radiation geometry for the antenna. It effects the SWR by eliminating a radiation source (the coax) that will reflect some of the radiated energy and show up as a part of the measured reflected power used in calculating SWR.

Yes, the use of a choke balun in base antennas is common/recommended.

[Alien Abductor]

how many wraps of the coax is usually done?

[Circuit Breaker]

From what I've seen and read, 6 to 8 turns.

If your SWR was 1.5, that isn't very bad. The only reason I could think of that you would need a choke balun is if you didn't have a ground plane for the 102 whip. The choke balun will certainly help keep the coax from radiating though and being a source of RFI.

[Red Warrior]

Remember the choke balun should be located as close to the feed point of the antenna as possible.

I have built dozens of them. Take a 7 " length of 4" plastic water pipe and wind 18 feet of RG8 or RG213 around it. Drill holes in each end to allow the coax to be tied down with cable ties. Solder the appropriate connectors on each end and connect to your antenna. Use a cable tie through the plastic to support the Balun. Do not allow it to hang by the coax.

Simple and effective.

[Alien Abductor]

10-4, ill be puttin up a station soon and want to do everything I can to keep the neighbors from callin the cops on me :r&r:

[linx]

I've got 8 turns, but the diameter of the turns is 6".

[DX47]

It is the lengh that is more important... do about 18ft at least. More won't hurt it, less will be less effective.

[Circuit Breaker]

I disagree...sort of. The choke works by having as much of the coax coiled by itself...the varying amplitudes of the signal traveling down the outside of the coax will also be at different phases and cancel each other out. The more turns, the better...which may require more length but that can also depend on the diameter of your coil. I've never done one before so I don't know if there's a formula or not.

[drdx]

I always run a choke at the base of my antennas. I started doing it long ago on a bumper mounted 102 inch whip that didn't want to behave and it helped. I've seen the 6-8 turn recommendation and the 4-6. I think the 6-8 I've seen was for use over the whole HF spectrum and we are way at the top 10% of it so the 4-6 may be plenty. They are cheap to do so while you're buying coax, just get the extra and make it. While I've seen fancy ones wound on forms, the ORR book just mentions coiling it up and ziptieing so I just do that. My current one is LMR400 under my puck mount and behind the headliner and insulated with foam to keep it quiet, so you don't see it without the map lamp removed.

The best part of the choke is its psychological effect. When one asks about an antenna issue I'll throw out the "what size is the choking on that?" routine and bla bla bla so they feel nice and clueless. It is probably doing me no good but I talk about choking like it is as essential as coax, and they believe it. Also, when you're down at the local donut shop comparing setups and ideas the choke gives you an extra edge of mystery. I can pop my map light off and shine a flashlight up there and they can see the choke. Then comes the "dang, dat boy is awl set up wit dat choke and awl. No wunda I hea him so fa. He runnin' that beeeg coax from ova at texas towas. U gotta be licensed to wauk in dea" I always have all of the fans running as well. I tell them they are temp controlled automatic. It is all in the illusion.

-drdx

['Doc]

About the only thing a coaxial choke is going to do is cut down, or stop and common mode currents on the outside of the feed line. They won't affect SWR much, if any at all. If they do, then you got problems somewhere else that ought to be corrected.
They will also tend to add a very slight amount of additional loss in the feed line because of the length of the coax used to wind that choke. That loss just isn't going to amount to anything at all, but it is there.
In general, adding a coaxial choke to a feed line isn't going to hurt anything. Not having one can result in some fairly weird things at times, or not make any noticeable difference. So, if you got the additional feed line to make one, why not? If not, then don't.
Where you put one can make a big difference. Near the feed point of the antenna is a very nice place if you're just using one of those chokes. It only 'chokes off' stuff after the choke, not before it. And then, additional chokes can be put other places till it get's really sort of ridiculous. All things considered, it's like cheap 'insurance', sort of...
- 'Doc

[4600 Turbo]

Is putting a Ugly balun the same as put up a 1:1 current balun?

['Doc]

"Is putting a Ugly balun the same as put up a 1:1 current balun?"

First, a choke, or what's usually called an 'ugly balun', isn't a balun at all. It won't 'help' a balanced to un-balanced situation except by 'choking' off the CMC on the outer surface of the coax. So an 'ugly balun' can do part of what a typical 1:1 balun can do, but not all of it.
So, an 'ugly-balun' is not a 1:1 balun, and using one doesn't mean you will get the results of using the other.
- 'Doc

['Doc]

The minimum number of windings in a choke will depend on what 'band' you want the thing to be functional on. They are frequency dependent just like any other coil is frequency dependent. And just like any other coil, the amount of reactance it has (resistance to AC) will depend on the number of turns and the diameter of the coil's form (also diameter of the conductor and a few other thingys). In general, or a very rough Rule Of Thumb (that's a 'ROT', btw, I like that one!) is half the band 'number' (11 meters = 5-6, 40 meters = 20, etc.) is the number of turns of some size roughly 4 to 12 inches in diameter. (I did say it was a rough estimate, didn't I? ) If it's a multi-band antenna, then go for the lowest band you think you'll ever put it on. When the length of coax making up the feed line gets to be a sizable percentage of the total feed line length, you've gone way too far, just 'fix' the problem where the problem exists. Chokes can certainly help, but they are very seldom a 'cure' for -the- problem. A coaxial choke only affects what's on the outside of the coax, not what's on the inside. If it affects what's on the inside of that coax, you've 'booger'ed the coax in some way when you wound the thing (typically too small of a coil diameter, who knows).
Do you have to wind a choke on a coil form? Not really. Just coiling it up like a rope will work just dandy in most cases. That's one reason why it's called an 'ugly balun'. Want to use some kind of coil form to make it look better? Great, knock yourself out! Won't hurt a bit! (Add a little glitter and you've created something that you can make ALL KINDS of lies about! That's a professional secret, don't tell everybody.)
- 'Doc