The AM Forum

I moved recently and the location of my ham shack and some large support trees would allow a fairly short feedline to an off-center-fed dipole. I'd be running at most 300W carrier on AM, and 1200W PEP on that "other mode"  :-[

The feedpoint impedance is close to 200 ohms, so a 4:1 balun is needed to match a 50 ohm coax. After reading L.B. Cebik's articles (and quite a few others) it appears that a current balun is required.

I could homebrew a Guanella balun but building a weatherproof enclosure would be a bit trickier (and the total weight might be an issue with it and the RG-8X coax hanging from the 80/20 feedpoint too). The W2DU (choke) baluns are available complete with PVC housing but I'm not sure its power handling capability would be adequate.  ???

Is there such a thing as 200 ohm window line, so I could run it down to the shack entrance and then put the balun indoors? The conductor spacing would have to be fairly close to get the Z0 that low, and a Google search didn't show any commercially available 200 ohm low-loss line.

Opinions welcome.
thanks
Charles
WB3JOK/0

A couple friends of mine use 450 ohm ladder line and have good results even thought the feed impedance is nowhere near 450 ohms.

why off center fed? at HF nobody can tell any difference in your length of cable. you could run 200' of rg 58 below 14 mhz. and no one would be the wiser. even if you do go off center fed, don't do it with coax. Recipe for epic fail. go open wire and use a tuner.

are you limited for space?

Someday 10m will open up again, and losses are more significant in small coax there...


Yes, I would not attempt to connect a 200 ohm balanced feedpoint with a 50 ohm (unbalanced) coax! But I am trying to avoid using a tuner. There are enough knobs to twiddle on my vintage gear without adding another set!
Besides, I don't own one  ::)

This OCF dipole (http://www.w8ji.com/windom_off_center_fed.htm) at W8JI's site looked like it would do nicely on any band in which I would be interested.


No, I have plenty of land (56 acres) but much is wooded. After looking over the cleared area around the house, it just looked like the feedpoint would be very near the second-story shack window using the existing trees as end supports. The multiband approach without traps or tuners is what I was really interested in, though.

thanks.
Charles

Hi Charles,

You might try wide spaced, 600 ohm feed line available from W7FG. Build that current balun but put it in the shack. Unbalanced to the balun and open feeders to the antenna. I haven't modeled it but I have seen this type of feed system used on center fed antennas. What do you think?

Mike

OCF Dipoles work well, and may fit your needs.  There are several commercially made OCF dipoles, which use a 200 ohm, 6-to-1 balun at the feedpoint, and use coax. 

This one is made by Butternut and is very popular.
http://hamcall.net/7bandocf.html
It is 95' on one leg and 45' on the other.

The Ham Call antenna looks good for what you want it to do for you Charles.

Not my cup of tea but I have problems remembering that other people don't like their tea the way I like mine, but that's not their problem. they like tea the way THEY like it. If I cant be helpful, I should STFU and mind my business. Sorry.  :-\

                                                                                                                                                                                                   

I think I'll build my own instead of buying it (will cost about $100 for wire, feeder and W2DU balun).

Don't apologize for providing helpful hints, 'DRB... not everyone may realize, for example, that high SWR on a coax feedline doesn't make for good radiated power due to the tremendous losses. Or that unless the feedline is an exact multiple of a half wavelength, further impedance transformations occur  8)

Anyhow I went ahead and ordered the W2DU 4:1 balun and will hang it at the feedpoint. Let's see how that works without a tuner - it looks good on paper at least and should get me on the air again.

Once I get the antenna hung I may try out my PW Command Sets (screen-modulated 10W carrier on 75 and 40m). Of course they don't have 50 ohm outputs so that's the next issue  ::)

-Charles

Maybe you could run two 450 ohm lines in parallel with a little spacing.

That's an idea, but they'd have to be well spaced (and the weight pulling down on the antenna would increase too)... plus it would cost twice as much...

I just discovered a very interesting LB Cebik article titled "How Much Coaxial Cable? A Case Study" in which he experimented with the identical configuration that I am building! (an 80/20 OCF dipole, 4:1 balun, coax feed).

It looks like it'll work just fine with the approximately 100' of RG-8X I will be feeding with, on 40, 20, 15, and 10 but NOT 75m :( (Nothing I'd read up to this point revealed that this otherwise good design only works without a tuner for the bottom 100 KHz of 80m)... SWR climbs really fast above 3600.

So if I want to operate on 3885 (or risk my sanity in 75m SSB belch-fart-world) I'll need a tuner anyway! At which point 300 or 450 ohm ladder line makes the most sense, and never mind the SWR on the feedline. Or put up another 135 foot wire antenna just for 75m. Or my 8 foot copper tubing magnetic-loop, which gives up a lot of gain...

-Charles

I gotz sanity?  ???

hey Derb ... which side of the bars are you on ?  Mysef, I dunno ...73 ...beefus

if you have 65 acres why are you messing with a H.A. antenna.

The best way to run the Windom is with a single piece of wire for the feedline.  That's the classic form of the antenna that dates back to the 30's or earlier.  Theoretically, it operates without standing waves on the feeder.  There is some feedline radiation, but the halfwave antenna still does most of the radiating.  You do need a fairly good ground connection though, but nothing like what is needed for an efficient vertical.

At least we are talking about baluns here, not "balums".  ;D ;D

Charles,
save your money on fancy balun's.  I use a homemade ocf dipole fed with 450 ladder line since '05.  Werks nice.  I couple my dx100 to a home made coax 1:1 current balun (IIRC about 20-25 ft.)  the othe rside goes to the input side of a  homebrew link coupled tuner.  Many people on here have made those.  Once you have a link coupled tuner you won't regret it.  No issues, simple and overall low loss, no heating up ferrite cores etc.  You will love the wide bandwidth of the ocf and lack of futzing with the tuner on 75-80 meters; also.   

have fun.

Jim

related info on....


http://www.bunkerofdoom.com/lit/bal_xmit/HFbaltransys.pdf

Thanks! I've read most of those articles separately, but it's handy to have them all in one file.

I do like the "no-tuner" dipole at the end of the document (actually it is tuned by switching different lengths of open-wire line into the feedline). I may try that one if the 80/20 OCF and W2DU balun don't do what I want them to.

My package from the Wireman just arrived so I can start measuring out 137 feet of springy wire  :D

-Charles

Three members of the local ham club (self-described "Old Farts")  ;D came over this morning with slingshot and fishing line, and we got the antenna hung from the trees. About 30-35' above ground, I think. Feedline from the balun goes straight down to the ground, so perpendicular to the antenna.

SWR at the rig is less than 2:1 on all bands except 75, and still only 3:1 there. So either I cut it a bit short, or the relatively low height above ground (and soil conductivity) is just "lucky" so the feedpoint impedance is a better match than it would be with the antenna in free space, or maybe the 100' of new RG-8X and the W2DU Balun are a bit lossier than advertised. Anyway the SB-102 tolerates it. Don't think I'll try the SB-200 at 3:1 though, might cause a zorch  :o

As soon as I tuned across 20m the first thing I heard was an S9 station - in Hawaii. Didn't get to work him though. It also works nicely on 40m. I got in the tail end of the 7.272 "72 Chew" group, and checked into the 7268 net also.

I wonder if I can bury the short section of feedline that is running across the lawn? Will the jacket of 8X tolerate burial?  ???

-Charles
WB3JOK/0

Chas,

Any coax will tolerate burial for a while as long as it is not too wet where you bury it!

Don't use the term RG-8X around Frank - it makes him nervous. If it is not RG-214 lab grade at 3 bucks a foot, well you are on your own!

I have had various grades of crap coax buried for years. But if you want to get religious, put it in some PVC pipe.

So what was the final configuration? Coax all the way to a balun off center? or Coax to a balun to open wire straight up?

Mike WU2D