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Author Topic: 1/4 wave feeder line vs random length 75 ohm feede  (Read 2270 times)

« on: July 27, 2002, 08:44:59 AM »

Hi, I have installed a 40 meter horizontal loop 140' total length up 15' high. It takes only 13' of feeder line to reach my rig. What would be the difference using a 1/4 wave of 75 ohm cable vs just enough 75 ohm cable to reach the rig?   I also will use the loop for 20 meters.   Bill

Posts: 1052

« Reply #1 on: July 27, 2002, 09:28:26 AM »

The difference DOES depend on the antenna....
1) What is the feed impedance?Huh?
2) Have you checked to the swr with staight 50 ohm feed???
3) As well as using the 75 ohm cable?Huh?
4) Have you found its reasonant point?Huh
5) Is it reasonant "where" you want it???
   6) Have you "pruned" it to be reasonant where you desire?Huh

NOT knowing the answers to the above questions, I can only give a "general" answer....

Bottom line:
Using a 1/4 wavelength of 75 ohm coax (~ 22' 8.5" at 7.15mhz, for "solid PE dialectric"/66% vel. fac.) will allow the typical loop impedance of 120 ohms to be "transformed" to 50 ohms, thereby allowing a VERY good match, easy load for your transceiver!!!!!

Using a 13' piece of 75 ohm coax will have SOME impedance "transformation" BUT, I'd have to get out a book or two, and do some math to figure exactly what the transformation would be........

BUT, the REAL bottom line is:
If the 13' piece works fine, then use it!!!(although in actual practice I'd recommend a few feet extra for future "remodeling of the shack", moving of the antenna, etc...)
If NOT use the 22' 8.5" piece!!!!!

HOWEVER....MY CONCERN is that having your antenna so close to your rig, you may find some rf feedback problems.....maybe not, but you should be ready to deal with them if they do occur....

ALSO, I'd recommend using a good quality "choke-type balun" or a "current balun".......Radio Works and The Wireman both make/sell excellent ones......
{some hams will simply say to "wrap the coax up in a coil at the feedpoint..."  But that is NOT a very good "balun"...and at lower freq. (especially <5mhz) that does NOT have enough inductance to do any good at all.....}
Uisng a balun on a balanced antenna, when using un-balanced feedline is always a good idea.....
(a loop IS a balanced antenna and coax IS an un-balanced feedline....)

Good luck.
John,   KA4WJA


Posts: 1052

« Reply #2 on: July 27, 2002, 09:47:21 AM »

Forgot the last part about 20 meters...sorry...

A 1/4 wavelength on 40m is a 1/2 wavelength on 20m....
(depending on the exact part of the band...but using the 7.1mhz - 7.15mhz section will give you a 1/2 wavelength at 14.2mhz - 14.3mhz....)

And since a 1/2 wavelength piece of transmiision line will show you the same impedance on the output as it has on the impedance "transformation" will occur......
EX: on 20m, your 140' horizontal loop may have a feed impedance of 100  -j200 ohms (off the top of my head, so NO flames from the purists...) and at the end of a 1/2 wavelength of transmission line it would show the same impedance...(allowing for some "loss" and assuming you were measuring at exactly the correct freq....)

      As far as what a 13' piece of 75 ohm coax would do?Huh??  As I wrote above, I'd need to break out a book and do some math......
But, my recommendations still hold.....

Use the 22' 8.5" piece and you'll be fine!!!!

Good luck.
John,   KA4WJA

P.S.  I read a few years ago, that "moving" the feedpoint of a mult-wavelength loop (such as your 140' loop on 20m) will affect the feed impedance....BUT not by too much.....I think the range was from around
- j100ohms......
So, I'd still recommend the LONGER it would allow you to "experiment" some......

« Reply #3 on: July 27, 2002, 10:00:04 AM »

Thanks John for all your answers and I am going to try to get the 22'- 8 1/2" peice of RG-59U cable from somewhere?  It seems that all the Ham shops around here don't carry it any more, not even Radio Shack. The cable made for TV only has an aluminum sheild so I have to be careful on what I order. Probably AES will be my last hope to find it.  Thanks again John for your help,  Bill

Posts: 1052

« Reply #4 on: July 27, 2002, 11:13:22 AM »

I got my RG-11 from "The Wireman".....
I think he's got RG-59 "copper" as well as RG-11...

They're also VERY nice to do business with!!

John,  KA4WJA

Posts: 3585

« Reply #5 on: July 27, 2002, 11:17:03 AM »

My local Radio Shack has RG11 foam in stock - and any can get it. The propagation velocity is usually 0.82 (for a quarter wave 234 divided by frequency in mHz, multiplied by 0.82) which means it will take a bit more than for solid poly coax and have just a teeny and utterly imperceptable bit less loss.

73  Pete Allen  AC5E

« Reply #6 on: July 28, 2002, 09:18:46 PM »

While I was using a random length of RG-58 coax for my 40 meter loop, my SWR was about 1:6. When I changed to a 1/4 wave feeder of RG-59U the SWR went FLAT. The loop was 140' total length and 15' high. It was fed from a corner. The feeder line was 22'  8 1/2" of RG-59U with a random lenth of 50 ohm coax enough to reach the rig.  Thanks for the help on this one.   Bill

Posts: 2193

« Reply #7 on: July 29, 2002, 10:27:39 AM »

Unless I am confused, the answer is simple. If you're using 75 ohm line, use 1/2 wave length (yes, 1/2 not 1/4). This will repeat the impedance at the other end. If your antenna was say, 50 ohms, that's what you'd see at the end of a 1/2 line, no matter what the impedance of the line. With some random length line, it would depend on the length and the terminating impedance.
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