## Re: [loopantennas] Re: 100Ω -> 50Ω balun?

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• ... Transformer (and balun) impedance ratios are proportional to the square of the turns ratios. Using only half of a winding reduces the impedance by a
Message 1 of 10 , Mar 8, 2013
On 03/08/2013 02:45 AM, ceo@... wrote:
>
> I'm using http://www.mingtek.tw/pdf/B-4.pdf
>
> 100Î© balanced fed into pins 14 and 16
>
> 50Î© output to R8 from pin 1
>
> pins 4 and 13 grounded
>
> Feel free to tell me I got it all wrong, I would not be
> at all surprised.

Transformer (and balun) impedance ratios are proportional to
the square of the turns ratios. Using only half of a
winding reduces the impedance by a factor of 1/4.

In concept, since a 1:2 impedance ratio requires a turns
ratio of 1:sqrt(2), that impedance ratio can be only
approximated.

A turns ratio of 3:2 gives an impedance ratio of 9:4 or

A turns ratio of 4:3 gives an impedance ratio of 16:9 or

A turns ratio of 5:3 gives an impedance ratio of 25:9 or

A turns ratio of 5:4 gives an impedance ratio of 25:16 or

etc.

--
Regards,

John Popelish
• That coupled with the fact that the receiver probably doesn t have an exact 50 Ohm input Z, and of course the loop antenna Z is very likely to change over a
Message 2 of 10 , Mar 8, 2013
That coupled with the fact that the receiver probably doesn't have an
exact 50 Ohm input Z, and of course the loop antenna Z is very likely to
change over a fairly wide range anyway. So I sure wouldn't worry about
some mismatch.

I have a similar setup here where I have an 88 Foot long dipole fed with
ladder line (the best I can do here). It feeds an oversized 2KW 4:1
balun (oversized to minimize heating losses from changing the balun
characteristics) before hitting the transmatch (MFJ949E). The balun is
primarily there for balanced feed as I realize the impedance varies a
great deal over the 3.5-29MHz range I use it. But it DOES perform very
well on all HF bands (and I've even used it on 6M too). I've tried MANY
different HF antennas in this Mobile home park (NO real estate space to
speak of) and this one outperforms everything else I've tried.

Yes, some verticals were a bit better on DX (especially a Butternut HF2V
I tried on 80/40), but their noise was WAY too high here.

So, I'd suggest just trying whatever matching ratio you have and see
what works best for you overall.

73 de Phil, KO6BB
http://www.qsl.net/ko6bb/ (Web Page)

Grundigs: S-350 (2006)& G6 (2011).
Icom: R-75 with Cascaded 250Hz CW Filters.
Kenwood: TS130S HF Transceiver (circa 1980).
Radio Shack: DX-380 digital portable (circa 1990).
Zenith: Royal-7000 Transoceanic (circa 1969).

Audio Interface: Two Mark One Earholes.
Decoder Software: Gray-matter between two ears.

ACCESSORIES: Homebrew LF-MF Pre-Amp, MFJ-949E HF Tuner
Homebrew 6 Hz Audio Filter.

ANTENNAS: 88' Long Ladder-line fed dipole at 35 feet AGL.
Ratzlaff 3 Foot Active Whip at 36 Feet AGL for LF/MW
4 Foot/side Tuned Rotatable Loop at 15 Feet AGL for LF

Merced, Central California, 37, 18, 37N 120, 30, 6W CM97rh

On 3/8/2013 5:02 PM, John Popelish wrote:
> On 03/08/2013 02:45 AM, ceo@... wrote:
>> I'm using http://www.mingtek.tw/pdf/B-4.pdf
>>
>> 100Î© balanced fed into pins 14 and 16
>>
>> 50Î© output to R8 from pin 1
>>
>> pins 4 and 13 grounded
>>
>> Feel free to tell me I got it all wrong, I would not be
>> at all surprised.
> Transformer (and balun) impedance ratios are proportional to
> the square of the turns ratios. Using only half of a
> winding reduces the impedance by a factor of 1/4.
>
> In concept, since a 1:2 impedance ratio requires a turns
> ratio of 1:sqrt(2), that impedance ratio can be only
> approximated.
>
> A turns ratio of 3:2 gives an impedance ratio of 9:4 or
>
> A turns ratio of 4:3 gives an impedance ratio of 16:9 or
>
> A turns ratio of 5:3 gives an impedance ratio of 25:9 or
>
> A turns ratio of 5:4 gives an impedance ratio of 25:16 or
>
> etc.
>
• ... After installing a 11 MB Chinese font file :-), I find there are _five_ transfomer diagrams there - you don t tell us which you use. ... Your pinouts only
Message 3 of 10 , Mar 8, 2013
--- In loopantennas@yahoogroups.com, John Popelish <jpopelish@...> wrote:
> On 03/08/2013 02:45 AM, ceo@... wrote:
> > I'm using http://www.mingtek.tw/pdf/B-4.pdf

After installing a 11 MB Chinese font file :-),
I find there are _five_ transfomer diagrams there
- you don't tell us which you use.

> > 100ÃÂ© balanced fed into pins 14 and 16
> > 50ÃÂ© output to R8 from pin 1
> > pins 4 and 13 grounded

Your pinouts only make any sense for two of them:
HN1664CG & HN1674CG. Let's hope it's the former.

> Transformer (and balun) impedance ratios are proportional to
> the square of the turns ratios. Using only half of a
> winding reduces the impedance by a factor of 1/4.

John has explained it well. A centre-tapped (usually
bi-filar) balun transformer is the way to connect an
FM dipole made of 300R twin feeder to the 75R coax
antenna input on the typical home hi-fi.

4:1 impedance ratio: half the voltage at 2x the current,
or 100R:25R. Also, if you are unlucky enough to have a
HN1674CG, you have the inductance of a half-connected
and un-specified current balun to add into the mix.

you could achieve a 3:2 or 4:3 turns ratio for one
of John's 'near miss' matches, but I imagine these
commodity transformers are inscrutably potted in
resin for their Ethernet isolation task.

However, does it matter for a receiver? There are
no damaging consequences of mismatch as in a Tx,
the coupling loop probably isn't 100R either, and
the only result might be transmission line
transformer effects when the cable gets to be
near an odd number of quarter wavelengths.

Better an antenna that works (mostly) than none.

Regards. LenW
• ... I think you should compare the result you get with one of these Ethernet baluns with a home made unit made of 5 strands of something like 30 ga wire wrap
Message 4 of 10 , Mar 8, 2013
On 03/08/2013 04:30 PM, hejsek2005 wrote:

> John has explained it well. A centre-tapped (usually
> bi-filar) balun transformer is the way to connect an
> FM dipole made of 300R twin feeder to the 75R coax
> antenna input on the typical home hi-fi.
>
> 4:1 impedance ratio: half the voltage at 2x the current,
> or 100R:25R. Also, if you are unlucky enough to have a
> HN1674CG, you have the inductance of a half-connected
> and un-specified current balun to add into the mix.
>
> you could achieve a 3:2 or 4:3 turns ratio for one
> of John's 'near miss' matches, but I imagine these
> commodity transformers are inscrutably potted in
> resin for their Ethernet isolation task.
>
> However, does it matter for a receiver? There are
> no damaging consequences of mismatch as in a Tx,
> the coupling loop probably isn't 100R either, and
> the only result might be transmission line
> transformer effects when the cable gets to be
> near an odd number of quarter wavelengths.
>
> Better an antenna that works (mostly) than none.

I think you should compare the result you get with one of
strands of something like 30 ga wire wrap wire, twisted
together and wrapped a few times through a ferrite core.

Then you connect all those strands in series. The first 4
of them form the 100 ohm primary and the last 3 form the
secondary. Two of those strands are part of both the
primary and also part of the secondary. The end of strand 2
(which is the beginning of strand 3) is the ground point.

This arrangement gives you a 16:9 = 100:56 ohm balun.

--
Regards,

John Popelish
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