[Boatanchors] HF Receiving Balun

Sun Mar 15 14:05:28 EDT 2009

Actually, the 9:1 un-un has become one of my favorites.  This transforms
50-ohm coax to 450 ohms.  Now, 450 is a good approximation of a
random-wire antenna, being the geometric mean of a 50-ohm'ish
quarter-wave wire and a 3000-ohm'ish half-wave.  ( SQRT (50 x 3000) =
387 )  This is the value assumed for short-wave in the design of the
golden-era all-wave broadcast and communications receivers, and 400
ohms  is value used in the RMA "dummy antenna" (see the SM-35/URM 25
Antenna Simulator)  This is also the basis of the "Magnet Long-Wire
Balun," which ain't a balun at all.

I chose mix-43 ferrite, the recommended material for MF/HF broadband
transformers, but something like mix-73 or 77 is probably a reasonable
facsimile.  The appropriate magnetic material makes a big difference in
these things.

Al

Carl wrote:
> I strongly agree with Al. The TV baluns start to fall off rapidly below 
> 10-12 mHz and most are pretty useless below 5 mHz according to VNA tests 
> that I have run on a large number of different versions.
>
> Some receivers are especially fussy about antenna impedances as they do 
> not have antenna trimmers or are purpose designed. Others such as the 
> Hammarlunds are more forgiving.
>
> My own baluns are 9:1 as MDS tests show a slight improvement but its 
> probably more for talking points than actual use. A balanced to balanced 
> tuneable LP style (pi net) passive preselector before the balun also 
> helps with some antenna configurations and also improves image 
> rejections on the old single IF radios.
>
> Binocular cores are great but a small parallel stack of regular ferrites 
> beads or toroids epoxied together works just as well if those are more 
> readily available.
>
> Broadband receiver baluns should use a ferrite mix that would be used at 
> a lower frequency in a tuned circuit application. In this case a 73 mix 
> for a balun core or a stack of beads, and 77 mix if using regular 
> toroids. This results in less capacitance between turns and a broader 
> plus flatter frequency response.
>
> In high power transmitter use different rules apply and powdered iron is 
> preferred in voltage baluns and tuned circuits.
>
> Carl
> KM1H
>
> PS: I just noted that Al specifies 73 mix in the article.
>
>
>
>
> ----- Original Message ----- 
> From: "Al Klase" <al at ar88.net>
> To: "Byron Tatum" <bjtatum1 at att.net>
> Cc: <Boatanchors at mailman.qth.net>
> Sent: Sunday, March 15, 2009 10:14 AM
> Subject: Re: [Boatanchors] HF Receiving Balun
>
>
>   
>> Hi Byron,
>>
>> Go to this page on my site and follow the "BALUNS" link:
>> http://www.skywaves.ar88.net/ANTENNA/antsys.htm
>>
>> Core material is very important.  E.g., TV baluns roll off below 5-10
>> MHz.  If you round up some of the BN-43-202 "binocular" cores, my
>> designs are good from about 100Khz - 60MHz.  In any event the winding
>> topology info should be what you need to know.
>>
>> Hope this helps,
>> Al
>>
>> Byron Tatum wrote:
>>     
>>> Hello-
>>>     I recall an article by Mr. Glenn Zook about using a TV-type 
>>> receiving balun with older shortwave receivers (such as HQ-129X) that 
>>> have around 300 ohms input impedance. The balun would noticably 
>>> improve reception.
>>>     I have just finished restoring and re-capping a HQ-129X. I am 
>>> thinking of aligning it using signal generator fed through the balun.
>>>     I would like to build my own as I have a lot of torroid and balun 
>>> cores for QRP use.
>>>     Does anyone know of a good construction article showing how to 
>>> build such a receiving balun?
>>>                                                     Thanks,
>>>                                                             Byron 
>>> WA5THJ
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>> -- 
>> Al Klase - N3FRQ
>> Jersey City, NJ
>> http://www.skywaves.ar88.net/
>>
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-- 
Al Klase - N3FRQ
Jersey City, NJ
http://www.skywaves.ar88.net/