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Thread: Connecting Vintage Hi-Z Receiver to Wire Antenna through Coax and Baluns?

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  1. #1

    Default Connecting Vintage Hi-Z Receiver to Wire Antenna through Coax and Baluns?

    Need your suggestions on matching end fed wire receiving antennas to various vintage BA type receivers. Most here have high-Z inputs (Hallicrafters, Nationals, Bretings, etc.) but also have a few Collins rigs with 50 ohm connections so will need capability to conveniently connect to both SO-239 and screw type terminals. Am planning to erect the antenna away from the house to minimize noise. If I connect 50 ohm coax to the antenna through a 9:1 balun what are your recommendations for connecting the coax to the receiver? A PL-259 would be ready to go on a 50 ohm input and the impedance would match but how would you handle the impedance mismatch connecting to screw type terminals on a Hi-Z input BA? Antenna tuner or a 1:9 balun possibly?

    Recently moved to a new QTH out in the country on 2 acres and on a temporary attic antenna I have loud S9 level electrical noise interference everywhere from BC up to about 6 mhz due to wireless computer networks and the usual assortment of electrical devices all over the place. The closest trees are just over 100 feet from the house and I thought maybe I should try to get a 100' wire antenna out away from the house and run shielded lead-in to the receivers. I would like to minimize noise while not compromising weak signal performance.

    Testing with a transistor radio shows no noise just outside the house.

    Thanks, Greg

  2. #2

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    For just receiving I never worried about antenna Impedance.

    When I was younger I would just connect the end of the antenna wire directly to the antenna input along with a homemade spark arrester
    and the other terminal to a good ground.
    Didn't even use a coax or feed line.

    then I would use the antenna trim (Built into My SX-111) to peak the received signal.
    could do the same thing to a radio that doesn't have an antenna trim by adding a variable capacitor inline with the antenna wire.

    Of course if you ever planned to use it for transmit that's a whole other matter
    I thought I could get rich in the stock market by investing in
    Viagra, Geritol, And Ginseng but my stocks didn't rise to the occasion.
    Hell, I figured that there was always somebody somewhere either trying to
    Get it up
    Get it going
    or
    Trying to remember what to do with it.
    73 De Bubba
    [COLOR=#0000CD]4 out of 5 Seniors prefer the taste of
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  3. #3
    Join Date
    Apr 2001
    Location
    north central Connecticut
    Posts
    1,106

    Default

    A 9:1 balun will minimize the SWR excursions at the coax, since the antenna impedance will vary widely across the HF SW range.
    The balun will lower the antenna impedance, reducing coax loss.

    The problem is that you are using an unbalanced antenna, and noise from the house can flow on the outside of the
    coax braid back to the antenna. You may have to install a ground rod at the balun, and then use a decoupling 1:1 balun
    on the feedline to eliminate RF noise. Try it and see.

    Don't worry about matching the receiver. Very few receivers have a well defined input impedance and the mismatch
    from going from 50 ohm coax to a high Z input isn't worth worrying about.

    Pete k1zjh

  4. #4

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    Many of the older receivers will definitely have reduced sensitivity caused by connecting an unbalanced relatively low impedance load to a high impedance balanced input. This effect is very pronounced on the Collins 75A-1 and to a lesser amount on many of the other "boat anchor" receivers. I have found that just by using a TV balun helps match the impedance very well.

    Although baluns made decades ago often didn't work well below around 40 MHz, baluns sold these days have to make it all the way down to 5 MHz because cable TV systems use frequencies that low for various things. Most of those baluns make it all the way down to the AM broadcast band.

    Here is an example of such a balun that is very inexpensive (50 cents in single quantities):

    http://www.parts-express.com/pe/show...number=180-010

    Just adapt the "F" connector to an SO-239.

    The article that I wrote a number of years ago about using TV baluns can be found at

    http://k9sth.com/uploads/TV_baluns.pdf

    Glen, K9STH

  5. #5

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    Quote Originally Posted by K1ZJH View Post
    A 9:1 balun will minimize the SWR excursions at the coax, since the antenna impedance will vary widely across the HF SW range.
    The balun will lower the antenna impedance, reducing coax loss.

    The problem is that you are using an unbalanced antenna, and noise from the house can flow on the outside of the
    coax braid back to the antenna. You may have to install a ground rod at the balun, and then use a decoupling 1:1 balun
    on the feedline to eliminate RF noise. Try it and see.

    Don't worry about matching the receiver. Very few receivers have a well defined input impedance and the mismatch
    from going from 50 ohm coax to a high Z input isn't worth worrying about.

    Pete k1zjh
    I agree, although I've never owned any really, really old receivers. But I've owned a lot made in the 50s, and a couple made in the 40s, and just connected 50 Ohm coax (which were connected to 50 Ohm or so antennas) to them and they worked just perfectly. Most had a ground strap allowing you to ground one of the two "screw" terminals to the chassis, for use with unbalanced lines.

    Many end-fed wires pick up more noise by design so are subject to noise problems not normally found with balanced antennas.
    What if soy milk is just regular milk introducing itself in Spanish?

  6. #6

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    An end fed wire will have a feed impedance of well under 50 Ohms to several thousand depending upon its electrical length at any particular frequency. Feeding direct with 100' + of coax will result in severe VSWR loss which can be determined with on line calculators.

    All a balun does is provide a match somewhere that it sees 450 Ohms and maybe a 2:1 range either side. It is NOT 9:1 when its looking into thousands of Ohms or 50 Ohms. Try it and see how it works but you will need another balun at the receiver end as a low impedance input will seriously degrade many if not all of the old balanced input radios. Their single wire feed inputs are a huge compromise also. A few have front panel antenna trimmers that can compensate a bit but they are not a do-all answer, even the HRO-60 suffers.

    I wouldnt waste my time with TV baluns. They state a low end of 5 MHz but many fall apart in the 7-9MHz region, they simply dont have enough ferrite mu to be effective. Its so easy to wind a good one anyway for pennies that will cover from below the BCB to 30 MHz. I use BN73-202 binocular cores and wind as seperate winding transformers to break up common mode nd with attention to detail measure 30-40dB isolation. Using one wire in autotransformer configuration will not do that.

    Ive a few RME DB-22A's that are used with various receivers for SWLing, they will take a single wire or balanced line input and output to a balanced line. Just what you need and added image rejection is a bonus. The DB-20 is a noise box and the DB-23 is a broadband IMD generator.

    An open wire balanced line fed long wire will minimize noise pickup on the feed if the unused wire is terminated in a good RF ground and VSWR loss will be a lot less. A mini ATU (MFJ, etc) and a 4 or 9:1 balun then feeds the radios.

    The first step should be buying a lot of 31 mix 2.4" ferrite toroids and start taming the noise generators in the house. It took me awhile and a bit of $$ but I now have a noise free house and can enjoy the radios. The neighbors are far enough away as to be almost in the noise.

    This has some good info and some not so good but the 31 mix stuff is good.
    http://www.audiosystemsgroup.com/RFI-Ham.pdf

    Carl

  7. #7

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    Many thanks for the helpful suggestions. Looks like I will need to identify and start fixing noise generators also but my priority for now is to erect a good receiving antenna away from the house as I just can't hear much right now on the attic antenna. I am thinking of attaching a support to a second story roof joist and start a 100’ wire several feet away and attach the other end to a tall tree over 100 feet out. The 9:1 lead-in balun would be close enough to the house to keep coax run reasonably short but measuring indicates it will still be about a 45’ run from the receiver location out to the balun attachment point. Will insertion losses be a concern if I add another impedance matching device at the receiver input? OK Carl on the DB-22A. I guess I will be looking into that as well. Someone recommended an MFJ-956 receiver antenna tuner, but a google search does not turn up many favorable comments at least in concert with what I am trying to accomplish.

  8. #8

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    Greg, some loss is tolerable at the lower frequencies where the noise level masks useable sensitivity anway. On 20 and up the loss not only increases but now starts to affect the receiver. Experiment with a balun at the receiver end, it may, or may not help. If this antenna is just for ham bands ther are "magic" lengths for all band dipoles and open wire feeders.

    For strictly SWLing the T2FD aint half bad as long as you use the longest one. The preselector will make up for the antenna signal loss to some degree.

    Try a 250W or even a KW Johnson Matchbox, they do well even a bit away from the traditional 80-10M bands.

    Carl

  9. #9
    Join Date
    Mar 2001
    Location
    Bremerton, WA
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    4,015

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    When I started SWLing in the mid-50s, my only antenna was a piece of #22 bell wire running diagonally across the ceiling of my bedroom, dropping down to my desk along the far wall, where my SX-99 sat. With this skywire, I managed to hear and get QSLs from all continents except Antarctica, and many of the states on AM BC, from Las Vegas NV. It's all I needed; I can't see anybody else needing much more than that.

    Of course, this was during the truly magical era when AM shortwave was the Internet, and propagation was phenomenal.
    Semper ubi sub ubi. 73
    K7KBN CWO4 USNR Ret.

  10. #10

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    I also had no trouble at all with a wire strung up in a room at the old QTH, that location was fairly quiet electrically and I could hear about everything quite well with simple antennas. In fact many times if I wanted to try a different receiver I simply connected a 10' piece of random wire and I was in business but that won't work here. Electrical noise interference is much greater at the new QTH which is surprising because we are out in the country now. The house was built in 09 so we probably have more noise generators now too which will have to be dealt with one at a time as Carl suggested. Even the washer and drier have circuit boards for example so not to mention the wireless networks. and light controllers. I am thinking first order of business is locate the antenna away from the house and use a grounded shielded lead-in to try to eliminate common mode interference from devices in the house. I am encouraged to try this simply from results with a portable transistor radio. Cannot hear even a strong local inside the house because of being swamped by noise but just stepping outside it gets quiet and receives lots of loud signals all across the band.

    Greg

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