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KC8YHU
04-24-2004, 11:21 PM
I need to know what type of balun I should use for converting my longwire with a good ground to coax. Should I use a 4:1 or a 1:1 or something else?

K5CEY
04-25-2004, 12:26 AM
You might do well to first fully describe your "longwire", what type of grounding system you have available, and what bands you plan to operate. Then I'll bet you'll get some ideas for a proper matching system.
# # # # # # # # # John

W5ALT
04-25-2004, 01:21 AM
Well, a balun is used used to convert from balanced to unbalanced transmission line. Your rig or tuner likely has a coax output, which is unbalanced. A long wire is unbalanced. Coax is unbalanced.

Why are you thinking of using a balun? Make sure you have an excellent ground and go for it. If you don't have a good ground, a balun won't correct that - you need to use something other than a long wire.

73,

K7FE
04-25-2004, 06:47 AM
The simple answer is: #You need an antenna tuner to match your very high impedance long wire antenna to 50 ohm coax, not a balun.

In the special case where you are using a single band multi-wavelength long wire and feed it 1/4 wavelenth from the end, then a 4:1 balun will usually allow a match to 50 ohm coax. #

One use for a balun is to match impedance, another is to reduce common mode currents from the shield of your coax or to go from balanced to unbalanced line or antenna's. #An impedance matching example might be if your antenna was 200 ohm impedance, then a 4:1 balun would match that to 50 ohm coax. #You may want to go the other way, say with a 12 ohm impedance mobil antenna then you would use a 4:1 balun in reverse which would yield 48 ohms which would match fine to the 50 ohm coax.

In your situation, you have not yet describe if this is to be used as one band long wire and if it is cut for that band or random length, or is this for multiband, so it is impossible to advise you on what it takes to perfectly match your antenna. #

I will however take a guess that your long wire is of random length, end fed, that it's impedance will be very high and that you wish to use it on several HF bands. #Given that, your antenna will be much greater feed impedance than 50 ohms. #Would a 4:1 balun help? #Well it won't match a 500 - 2000 ohm antenna to 50 ohm coax very well, but if you had an antenna tuner that had limited impedance matching capabilities, then a 4:1 balun might bring the impedance closer to what a poorly designed antenna tuner might match. #

As far as using coax goes, I would not use it unless you have some RFI in the shack issues. #I like to bring the long wire all the way to the tuner so that I can match the antenna properly. #Some people use a remote antenna tuner outside to keep RF problems in the shack down and then run coax with ferrite beads on it, (or a 1:1 balun) from the remote tuner into the shack.


73,
Terry, K7FE

W8JI
04-25-2004, 12:10 PM
</span><table border="0" align="center" width="95%" cellpadding="3" cellspacing="1"><tr><td>Quote (kc8yhu @ April 24 2004,16:21)</td></tr><tr><td id="QUOTE">I need to know what type of balun I should use for converting my longwire with a good ground to coax. #Should I use a 4:1 or a 1:1 or something else?[/QUOTE]<span id='postcolor'>
If you have a less-than-perfect ground, a current balun or choke balun can help prevent RF in the shack. In this case it would act like a feedline isolator.

It will ONLY help if the antenna ground or counterpoise is not directly connected to the radio gear.

I explained some of that at

http://www.w8ji.com/verticals_and_baluns.htm

What applies to verticals with poor ground applies to longwires. A GOOD ground is very complex and difficult to install. Many poor or marginal grounds are often incorrectly thought of as good.

The key is if you have RF in the shack, you either need to change the antenna system or isolate the RF from the shack. Sometimes isolating the RF through use of a choke or current balun is easiest.

73 Tom

W0LPQ
04-25-2004, 02:02 PM
Agree with Tom and Terry. You need a tuner, not a balun. If you have RF in the shack then a current balun (10 turns about 8-10&quot; diameter of Coax) into the tuner will cure that.

I have multiple antennas and an antenna marix built of coax relays. Had RF getting into the TR-4 and current baluns off of each radio and antenna cured the problem.

73

Bill, W0LPQ

KL7JV
04-26-2004, 08:41 PM
K7FE I was just wondering how you know his long wire has a high impedence? It is possible to have a long wire with a low impedence.

AG3Y
04-26-2004, 09:02 PM
Several assumptions are being made here. First when YHU describes his antenna as a &quot;long wire&quot;, we assume that he is referring to a random length wire that is probably not resonant on any particular band of interest. A true &quot;long wire&quot; would more than likely be several wavelengths long on the major band of interest, and be cut to a specific length as to present a reasonable load to the rig. But that is hardly ever the case.

Secondly, he states that he does have a good ground, but this is often not the case. Therefore, he may have a fairly low impedance antenna, but normally, because the antenna is not working off a good ground, that is not necessarily so. A good ground allows RF currents to flow away from the shack, thus preventing problems normally associated with an unbalanced antenna system which occure without that good ground such as &quot;a hot chassis&quot;, &quot;RF in the shack&quot; etc.

I continue to believe that the best solution to any problems that occur with end-fed antennas is to feed it in the center &#33; Feedline is NOT that expensive or difficult to work with, especially open ladder-type line&#33;

73 from Jim AG3Y

VK4XJB
04-26-2004, 10:40 PM
A long wire with a #bit of thought can be low #impedence and easy to feed.

My long wire is setup for 10m and can be 300 feet or approx 820 feet in length depending on whether I want gain + still reasonable pattern or lots of gain in a single direction.

With the way it runs and the ground type here it is close to 150 ohm for the shorter length and close to 200 ohm for the full length. It is fed against ground. I use 50 ohm coax and a 4:1 transformer at the feed point. No tuner required. The wire needs to be resonant to get rid of any reactance.

Another method I have been using for many years is an inductor and capacitor in parallel across the feed point and resonate this at the desired frequency. The braid of the coax goes to the ground end of the coil. Run a small lead from the center of the coax to the point on the inductor that presents a good match. Try differnt points until a good match is found. I have used this to match non resonat wires. I have found this kind of match + a terminated long wire gives a wire with a nice bandwith and usefull front to back ratio.

W8JI
04-26-2004, 11:23 PM
</span><table border="0" align="center" width="95%" cellpadding="3" cellspacing="1"><tr><td>Quote (vk4xjb @ April 26 2004,15:40)</td></tr><tr><td id="QUOTE">A long wire with a #bit of thought can be low #impedence and easy to feed.

My long wire is setup for 10m and can be 300 feet or approx 820 feet in length depending on whether I want gain + still reasonable pattern or lots of gain in a single direction.

With the way it runs and the ground type here it is close to 150 ohm for the shorter length and close to 200 ohm for the full length. It is fed against ground. I use 50 ohm coax and a 4:1 transformer at the feed point. No tuner required. The wire needs to be resonant to get rid of any reactance.

Another method I have been using for many years is an inductor and capacitor in parallel across the feed point and resonate this at the desired frequency. The braid of the coax goes to the ground end of the coil. Run a small lead from the center of the coax to the point on the inductor that presents a good match. Try differnt points until a good match is found. I have used this to match non resonat wires. I have found this kind of match + a terminated long wire gives a wire with a nice bandwith and usefull front to back ratio.[/QUOTE]<span id='postcolor'>
A longwire with low impedance places the MAXIMUM demand on the ground system resistance to prevent &quot;RF in the shack&quot; from current and to promote efficiency.

A longwire with high impedance feed requires a less stringent ground, but them you have electric field issues from voltages that cause high displacement currents and RFI.

The best idea is to never bring the longwire into the shack, or better yet feed it as a dipole.

73 Tom

VK4XJB
04-28-2004, 01:06 AM
Matching my wires how I described does leave them outside. It also provides a good match to coax.

Many years ago I did try a wire inside and it was not fun. Distorted audio and my all time favourite, key up and have the radio go straight to full power and stay there. Since then they have stayed outside.

K0IZ
04-29-2004, 08:29 PM
There are a number of good web sites that discuss antenna issues, perhaps the best is http://www.cebik.com/radio.html He has some good info on end-fed wires and baluns.

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