Low noise antenna feedline

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Low noise antenna feedline

Post by Nulluser00 » Thu Aug 02, 2018 9:09 pm

In post: https://www.sdrplay.com/community/viewt ... f=5&t=3521 'glovisol' brought up a very important point.

"This article is representative of what could and should be done with a conventional receiver with umbalanced 50 Ohm input. But with this scheme the risk of the in between coax line becoming a noise antenna itself is ever present, so at best this and many more such schemes in the literature can work for one guy and make another unhappy. The comparatively inexpensive, but steel enclosed SDRplay unit, allows us to place the receiver and a preselector in any convenient place close to the antenna terminals, provided we use a buffered USB cable, and do away in one shot with all possible unwanted noise and interference sources, while we comfortably sit in the shack. Furthermore the balanced antenna input is a major factor in noise rejection right at the receiver's."

This is a guess, but I suspect 'glovisol' is a radio amateur radio operator and is primarily interested in listening to very narrow swaths of frequencies. Because I listen to everything from 10KHz to ~25MHz my needs are different then his.

When I ran into an EMI nightmare at my home after replacing the heat pump with a geothermal unit with variable speed drives my aunt, who is also my boss at the engineering consultancy where I work, gave me this link: "Is Your Coaxial Lead-In Actually an Antenna??" [John H. Bryant with Bill Bowers, February 2001, May, and November 2003] <link> shows one way to eliminate unwanted signal pick up on our coaxial cable feedline.

I copied the following graphic from "ON4UN's Low-Band DXing, 3rd edition."

Darn I can not attach images!
This might be it....
The graphic shows coaxial cable with 100 ferrite beads a the shield connected to a ground rod and another set of 100 beads.

Wireman offers the proper beads and they aren't terribly expensive....
912 FB 73-2401 Ferrite bead u=2500 for HF W2DU balun .30 each, .25ea/50, and, .20ea/100.

There is much less expensive option. Instead of several sets of 100 ferrite beads strung out on the coaxial cable, a toroid with turns of RG-179 75 ohm miniature coax is either as effective or very nearly effective.

Mr. Bryant used a FT-140-75 core with 19 turns of RG-174.

RG-174 and RG-179 are the same diameter.
RG-174 50 ohms 0.100
RG-179 75 ohms 0.100

RG-187 75 0hms 0.105 diameter, Teflon inner insulator, >>much more expensive<<, but a local ham had ~~100 feet he gave me in return for sharing my experiences with this antenna setup.

Martin shows a very similar technique on his Active Antenna page.
https://mediaprocessor.websimages.com/w ... 0balun.png
https://mediaprocessor.websimages.com/w ... tion-1.png
RG-174 could be substituted for the RG179B/U for use in a 50 ohm transmission line....

I allowed my aunt to read this before posting. She mentioned one major advantage of SDRs is their wide frequency coverage and thought a "preselector" might defeat that advantage.

I'd never heard of a "preselector" so she demonstrated one last night. Her first SW radio was a Heathkit GR54. This receiver covers the .530~30MHz in four bands. It is a single conversion unit with an IF of .455MHz. For frequencies above ~4MHz the single conversion allows "images" to be received. "Images" appear signals either .455MHz above of below the tuned signal.

A "preselector" is a set of tuned circuits that is placed between the antenna and receiver and it prevents signals other then the one the receiver is tuned to from reaching the receiver. My aunt's home made "preselector" has a 100~200KHz bandwidth with fairly sharp "skirts." The GR64 was a low end, beginners, radio. It even has a "Q-multiplier," but I'll let the reader look it up on their own. Those of us who came along recently have no idea just how good even our budget receivers are. My DX-398 literally blows the GR64 away. My aunt has rebuilt her first receiver several times and it works at least as good now as it did when it was new.

I love being able to enter a frequency and have my receiver "magically" tune to the desired frequency and allow me to see the adjacent signals.

I've mentioned W1HIS seminal work on Common Mode Chokes and it is well worth reading.

Some webpages with useful information...


http://valentfx.com/vanilla/discussion/ ... e-solution

https://web.archive.org/web/20161225095 ... l%2001.pdf

Last edited by Nulluser00 on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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Posts: 486
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Location: Cheshire, UK

Re: Low noise antenna feedline

Post by g1hbe » Fri Aug 03, 2018 7:47 am

Thanks for going to the trouble to post this Noel, it's all very useful info. Just one point - in a superhet with a 455 KHz IF, the image frequency will be at TWICE 455 KHz above the receiving frequency. This is because the local oscillator will be running at Fsignal + IF and the image response will be at LO + IF. So the image freq will be at Fsignal + 910KHz.

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