Warning: Creating default object from empty value in /homepages/11/d132647312/htdocs/Amfone/mkportal/include/SMF/smf_out.php on line 47
Help With Balun




 
The AM Forum
January 18, 2021, 05:52:39 PM *
Welcome, Guest. Please login or register.

Login with username, password and session length
 
   Home   Help Calendar Links Staff List Gallery Login Register  
Pages: [1]   Go Down
  Print  
Author Topic: Help With Balun  (Read 4084 times)
0 Members and 1 Guest are viewing this topic.
N0WVA
Member

Offline Offline

Posts: 291


« on: February 28, 2005, 11:21:58 PM »

I was wondering if some of you might help me, as I havent had much experience with baluns. My dad uses a CB to communicate between trucks and home. But he lives in a valley. I built a 2 element quad and would like to feed it with 300 ohm twinlead. We are going to put it up on the hill and use about 250 ft of feedline. I think he will get much better coverage this way, and twinlead will  keep losses low with the already piss weak transmitter.

The quad should be around 50-70 impedance with a .14 spacing between the elements.

I need to figure out a way to make transformers for each end of the twinlead. Would it be 6:1? Whats the best way to do this? With a toroid? Air core? Can I use PVC and magnet wire? Thanks.


Doug
Logged
KE1GF
Guest
« Reply #1 on: March 01, 2005, 09:03:14 AM »

Feed the quads with the twinlead and use it as an xfrmr, learn the smith chart in the back of the arrl handbook.

-Bill 'GF
Logged
Bacon, WA3WDR
Member

Offline Offline

Posts: 881



« Reply #2 on: March 01, 2005, 09:48:54 AM »

You could design a 1/4 wave matching section of balanced line.  The impedance would be the geometric mean of the 300 ohm twinlead impedance and the 50 to 70 ohm antenna impedance.  So if you are going to use 300 ohm line and the driven element is about 60 ohms, the necessary matching section impedance would be the square root of (300 * 60) or about 134 ohms.  You might just make a 1/4 wave (electrical) section of two 300 ohm lines in parallel, that would be 150 ohms and the mismatch would not be very bad.  Or you could calculate the necessary spacing for reasonable wire dimensions to make 134 ohm line, and then make a 1/4 wave matching section that way.

Remember that the velocity factor of twinlead depends very much on whether it is open-line or jacketed, and what the jacket material is.  But it is much higher than coax.  So the electrical 1/4 wave lines will be closer to 1/4 wave physical length than they would if you used coax.
Logged

Truth can be stranger than fiction.  But fiction can be pretty strange, too!
W2VW
Contributing
Member
*
Offline Offline

Posts: 3489


WWW
« Reply #3 on: March 01, 2005, 11:22:32 PM »

If your quad is already 50-70 Ohms then you could simply use a multiple of electrical 1/2 wavelengths of 300 Ohm stuff and  no  matching crap. Your twin lead will have a high SWR but will not radiate and lose power if it is installed properly. SWR at the transmitter end will be OK. Electrical 1/2 length means the feedline's velocity factor (stated in spec sheet for your type of line) in percent times 1/2 wavelength. .552 metres is 27.185 megs. When you use an electrical 1/2 wavelength of feedline, the feedpoint situation repeats itself. You might need a whole bunch of 1/2 waves to make the run to your antenna but it still works as long as the length is some multiple. If the antenna were reactive of needed some matching the situation would get more complex. 2 problems here. The feedline will change with weather and you have no way to re-tune if you are just hooking it to a chicken band rig. The velocity factor for your type of feedline might not be exact and you will have to juggle length.
There are better ways to do this without spending a lot of money. The world is full of cheap 1/2" foamflex hardline. C.A.T.V. uses this stuff from large rolls and they disgard amounts under a few hundred feet rather than splicing it. It is low loss but is 75 Ohm cable. Shows up at hamfesters all the time. Someone on this board probably could come up with a stash. Same thing goes though. use a multiple of an electrical 1/2 wavelength and the impedance at your shack will be what it is at the antenna. Connectors for this cable can be fabricated for cheap.
Logged
K1JJ
Contributing
Member
*
Offline Offline

Posts: 8495


"Let's go hiking in the woods, Tommy!" - Yaz


« Reply #4 on: March 01, 2005, 11:48:58 PM »

Hola,

I wouldn't use twin lead for the 250' run. That's a LONG run so will not be environmentally stable during rain and icing condix. The quad may still "work OK" with ice, but an iced balanced feedline will kill it completely, especially on a freq as high as 27 mc.

Take Dave's suggestion and seek out some CATV 75 ohm alum hardline. It's usually free after a few phones calls or visits to the right places. I use 5,000' of it here and love the stuff.

At the feedpoint, make up a 1/2 wavelength coaxial balun and connect the hardline directly to it and the element. Look in the Handbook for details of using a simple coaxial balun. I use them on 6M for Yagis here.
If you are stumped, I will describe the connections here for you. You can use RG-11/U 75 ohm coax for the balun. (About 12' long inc the velocity factor)  This will keep the whole system balanced - no need for twin lead.

Remember too, that you would need to support the twin lead for its complete run, isolated from other objects including going into the house, or inbalances will occur. Hardline can run anywhere.

Maybe you can adjust the quad's shorting stub to see 75 ohms - go for a perfect match if you can, but if 50 ohms is all you have, then a 1.5:1 swr will hardly matter with 75 ohm hardline.  

BTW, you will be seeing about 0.3db loss/100', or a total loss of about 0.75 db for the 250' run with a perfect match on 27 mc.

73,
Tom, K1JJ
Logged

Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed. 

Nothing like a new homebrew rig. Come into the shack, flip on the switches and everything works perfectly.

And, nothing like an old dog.
N0WVA
Member

Offline Offline

Posts: 291


« Reply #5 on: March 02, 2005, 12:32:18 AM »

Ok, thanks for all the replies. Ive had the books out doing some studying to go along with all of your suggestions.  I hardly ever operate 10 meters, or I might have already gone through all this trouble, thinking about line loss and stuff. But this episode with this CB has at least turned into a learning experience. And my shack has yet to go back together, with me buying a house and getting married. At least I have a corner picked out in the spare room.

Dave and Tom, you guys about have me talked into using the CATV stuff. The figures I had in my head about RG8x were kinda lossy. Looks like the cable stuff is better. And fat RG8 costs a small fortune.

I will inquire around about the hardline, but if some of you guys have some to spare, shoot me a price.

We just got the quad up a few feet today. Hung it from a rope over a limb. Fed it with a short run of rg58u and it has a swr of 1.5 give or take a little.
I just opened up the Driven Element and soldered on the coax. So at least I know its in the ballpark.

Looking for hardline.......

Doug
Logged
Pages: [1]   Go Up
  Print  
 
Jump to:  

AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
 AMfone 2001-2015
Powered by SMF 1.1.21 | SMF © 2015, Simple Machines
Page created in 0.086 seconds with 18 queries.