[TowerTalk] Dipole puzzle

Sat Oct 15 20:08:43 EDT 2005

You can configure a 4 to 1 current mode balun using two physically 
separated cores.  There is an example in ARRL Antenna Handbook 20th 
edition page 26-24.  Seems like you should be able to buy these also, 
but I have not searched for them.  If the one you have is this mode, 
then I suspect it is bad.

You can't measure the antenna resonant point with the balun removed, 
(unless you are at the antenna) because of the off center feed and the 
coax which has a big mismatch at one end.  The other end can read just 
about anything depending on coax length.

Jerry, K4SAV

K4SAV wrote:

>It's not your antenna wire.
>
>You don't have the right kind of balun(s).  You have two requirements 
>for the antenna,  You have to match the antenna impedance and you have 
>to take care of the feedline currents because of the off-center feed.  
>For the first you need a 4:1 voltage mode balun.  For the second, you 
>need a current mode balun.  I don't know if it is possible to have both 
>on one core or not, but I can find out if you can't.
>
>Because of the off center feed, your antenna right now consists of about 
>196 feet on one side, and on the other side, two connected wires.  One 
>is about 38 feet of antenna wire and the other is your coax (ever how 
>long that is). The coax shield is acting as the other half of your 
>antenna.  You would be extremely lucky if it was resonant on 160 meters.
>
>Jerry, K4SAV
>
>
>William Q Meeker wrote:
>
>  
>
>>I recently installed an off-center-fed dipole, cut for 1/2 WL on 
>>1.824 MHz (233 feet), using #12 Davis RF flexweave. I am feeding off 
>>center so that the feed point and transmission line will be in the 
>>backyard instead of the front yard! The front yard support is about 
>>50 feet high in a tree. The backyard support is about 70 feet high at 
>>the top of my tower. The distance between the supports is about 280 
>>feet and the dipole is centered between them with 4-inch insulators 
>>and rope on the ends.
>>
>>I choose the feed point to be about 36.82 feet from the tower end of 
>>the dipole, where EZNEC predicted Z=196+1.5j, a good match for my 4:1 
>>balun and 50 ohm coax. EZNEC also predicted other interesting points 
>>where the antenna would be resonant.
>>
>>After pulling up the antenna, I used my MFJ259B to check things out, 
>>expecting the need to fine-tune the location of the feed-point. I was 
>>surprised to see that I was measuring something close to the 
>>predicted results above 3MHz or so (e.g., a nice resonant point in 
>>the middle of the 80-meter band), but totally weird results in the 
>>160 meter band---R values on the order of 1 or 2 ohms (where EZNEC 
>>predictes R in the 100s'). I then brought my radio outside to confirm 
>>the results. Infinite SWR in the 160 meter band, but sensible things 
>>below 80 meters.
>>
>>I also pulled the balun out to see what would happen. Again, 
>>predictable results below 80 meters (above 3.5MHz) and weird results 
>>(R close to 0) in the 160 meter band.
>>
>>I came across the following in a recent TT posting, and am beginning 
>>to wonder if the flex weave might, for some reason, be a problem at 
>>low frequencies? It is working fine for inverted vees that I have up 
>>for 40 and 80 meters.
>>
>>       Braiding has significantly higher impedance than solid conductors.
>>       A textbook I have  on transmission lines says it is 3 to 4 
>>times worse than a solid
>>       conductor but that is for a transmission line lay that is 
>>clean and tightly pressed.
>>       Lays used in transmission lines are generally not at sharp angles
>>       and are not densely woven, so they have less resistance per 
>>unit length
>>       than a rapid weave that is at more at right angles to the current path.
>>
>>
>>I am contemplating two alternatives:
>>
>>1. Replacing the OCF dipole with an inverted vee with the apex on the 
>>tower at 65 feet or so (but EZNEC predicts the OCF dipole will be 3 
>>or 4 dB better that the inverted vee in the right directions and at 
>>the right angles).
>>
>>2. Replacing the flexweave with copperweld (the flexweave is a joy to 
>>work with, relative to copperweld, however---but I did note recent 
>>posting suggesting potential lack of longevity with some flexweaves).
>>
>>I would rather not have too many more failed experiments, so am 
>>seeking knowledge and advice.
>>
>>Bill
>>K0KT
>>
>>
>>
>>William Q. Meeker
>>Department of Statistics
>>304C Snedecor Hall
>>Iowa State University
>>Ames, Iowa 50011
>>Phone: 515-294-5336
>>Fax: 515-294-4040
>>Home Fax: 515-232-1323
>>www.public.iastate.edu/~wqmeeker 
>>
>>_______________________________________________
>>
>>See: http://www.mscomputer.com  for "Self Supporting Towers", "Wireless Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.
>>
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>>TowerTalk at contesting.com
>>http://lists.contesting.com/mailman/listinfo/towertalk
>>
>> 
>>
>>    
>>
>
>_______________________________________________
>
>See: http://www.mscomputer.com  for "Self Supporting Towers", "Wireless Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.
>
>_______________________________________________
>TowerTalk mailing list
>TowerTalk at contesting.com
>http://lists.contesting.com/mailman/listinfo/towertalk
>
>  
>