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OT- Using two coaxial cables as a balanced feeder

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john petters

OT- Using two coaxial cables as a balanced feeder

Sorry to take up bandwidth here, but as there are some very knowledgable
folk on list I thought I would run this query.

I've been playing with a balanced feeder for my quad - DJ4VM driven
element cut for 20M.

I have 30M reflector. The impedance is very low on 30M and it does not
like the 450 ohm line.

I got over this by putting a 9:1 step down transformer, which works OK,
but I suspect it may be lossy on the other bands where it is not needed.

I thought it would be a good idea to make a balanced line from two
pieces of RG213 to reduce the ohmic losses of the cable (the run of 450
ohm measures 6 ohms DC) and get a closer chacteristic impedance to the
the quad on 30M

I made two lengths of RG213 coax with the inner and outer of each cable
parralled and the two cables stuck together with insulating tape as a
balanced line This did not work at all well for some reason.

I have also (as per the RSGB Rad Com Handbook and seen nowhere else)
rearranged the cables so that the braids are connected together and
grounded with the two inners providing the balanced line.

On a small version of the DJ4VM quad as a test antenna, which is only a
couple of ohms on 20m, the K2 sounds dead. removing the braid and using
the feeder as a long wire brings the signals way up.

My query is this. Is there an error in principle in using two pieces of
coax as a balanced line, which would imply the handbook is wrong or am I
missing something.

I did wonder if the braid, used in this manner has a different velocity
factor to the inner and whether this could be the problem - or am I
trying to do the impossible?

I have experienced a related problem in the past when I used a smalll
loop antenna on 160M - about 25 ft on each leg, mounted vertically. The
botton being about three feet above ground. The design was by Paul
G4LNA as can be seen on the link below

http://www.geocities.com/g4lna/pages/myant.html

Fed with 50 ohm coax through a step down transformer with one side of
the secondary going though a series capactor to one side of the loop,
the other side of the transformer connected to the return of the loop.
This antenna worked very well on 160 - but if fed with 450ohm line
through a balanced ATU, did not radiate at all despite a low SWR.

Any ideas welcome off list.
73
--
John Petters
www.traditional-jazz.com
Amateur Radio Station G3YPZ
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Don Wilhelm-4

Re: OT- Using two coaxial cables as a balanced feeder

John,

Your pair of 50 ohm coax lines has twice the loss of a single coax line
and the characteristic impedance is 100 ohms, so I would expect its loss
to be much greater than using the 450 ohm line with no lumped
transformers (the feedline itself will act as an impedance transformer,
and a quite efficient one at that.

Can I suggest that you might use a length of 450 ohm (or better yet open
wire line) that is about a quarterwave at 30 meters as a matching
section. That will transform the input impedance to something much
higher, then run 450 ohm line to the shack.

It is not unusual to use 450 ohm ladder line with a widely varying range
of antenna impedances. Even when operated at high SWRs, the ladder line
(when dry) has a very low loss.

If your tuner has a limited range and cannot properly match the
feedpoint impedance of the feedline, adding (or subtracting) a length of
line to the feeder length can bring it into a range that your tuner can
match. I would suggest adding or subtracting in 1/8th wavelength
increments until you get it close to the desired range and then go to
1/16th wavelength additions or subtractions. The feedline acts like an
impedance transformer when it is operated with an SWR other than 1.

73,
Don W3FPR

john petters wrote:

> Sorry to take up bandwidth here, but as there are some very knowledgable
> folk on list I thought I would run this query.
>
> I've been playing with a balanced feeder for my quad - DJ4VM driven
> element cut for 20M.
>
> I have 30M reflector. The impedance is very low on 30M and it does not
> like the 450 ohm line.
>
> I got over this by putting a 9:1 step down transformer, which works OK,
> but I suspect it may be lossy on the other bands where it is not needed.
>
> I thought it would be a good idea to make a balanced line from two
> pieces of RG213 to reduce the ohmic losses of the cable (the run of 450
> ohm measures 6 ohms DC) and get a closer chacteristic impedance to the
> the quad on 30M
>
> I made two lengths of RG213 coax with the inner and outer of each cable
> parralled and the two cables stuck together with insulating tape as a
> balanced line This did not work at all well for some reason.
>
> I have also (as per the RSGB Rad Com Handbook and seen nowhere else)
> rearranged the cables so that the braids are connected together and
> grounded with the two inners providing the balanced line.
>
> On a small version of the DJ4VM quad as a test antenna, which is only a
> couple of ohms on 20m, the K2 sounds dead. removing the braid and using
> the feeder as a long wire brings the signals way up.
>
> My query is this. Is there an error in principle in using two pieces of
> coax as a balanced line, which would imply the handbook is wrong or am I
> missing something.
>
> I did wonder if the braid, used in this manner has a different velocity
> factor to the inner and whether this could be the problem - or am I
> trying to do the impossible?

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Don Wilhelm-4

Re: OT- Using two coaxial cables as a balanced feeder

Folks,

I just received a correction from a reliable source (Tom W8JI). The
loss in a parallel set of coaxial lines is not 2 times the loss for a
single line, but the same as a single line.

I believe my other points still hold true.

73,
Don W3FPR

Don Wilhelm wrote:

> John,
>
> Your pair of 50 ohm coax lines has twice the loss of a single coax line
> and the characteristic impedance is 100 ohms, so I would expect its loss
> to be much greater than using the 450 ohm line with no lumped
> transformers (the feedline itself will act as an impedance transformer,
> and a quite efficient one at that.
>
> Can I suggest that you might use a length of 450 ohm (or better yet open
> wire line) that is about a quarterwave at 30 meters as a matching
> section. That will transform the input impedance to something much
> higher, then run 450 ohm line to the shack.
>
> It is not unusual to use 450 ohm ladder line with a widely varying range
> of antenna impedances. Even when operated at high SWRs, the ladder line
> (when dry) has a very low loss.
>
> If your tuner has a limited range and cannot properly match the
> feedpoint impedance of the feedline, adding (or subtracting) a length of
> line to the feeder length can bring it into a range that your tuner can
> match. I would suggest adding or subtracting in 1/8th wavelength
> increments until you get it close to the desired range and then go to
> 1/16th wavelength additions or subtractions. The feedline acts like an
> impedance transformer when it is operated with an SWR other than 1.
>
> 73,
> Don W3FPR
>
> john petters wrote:
>> Sorry to take up bandwidth here, but as there are some very
>> knowledgable folk on list I thought I would run this query.
>>
>> I've been playing with a balanced feeder for my quad - DJ4VM driven
>> element cut for 20M.
>>
>> I have 30M reflector. The impedance is very low on 30M and it does not
>> like the 450 ohm line.
>>
>> I got over this by putting a 9:1 step down transformer, which works
>> OK, but I suspect it may be lossy on the other bands where it is not
>> needed.
>>
>> I thought it would be a good idea to make a balanced line from two
>> pieces of RG213 to reduce the ohmic losses of the cable (the run of
>> 450 ohm measures 6 ohms DC) and get a closer chacteristic impedance to
>> the the quad on 30M
>>
>> I made two lengths of RG213 coax with the inner and outer of each
>> cable parralled and the two cables stuck together with insulating tape
>> as a balanced line This did not work at all well for some reason.
>>
>> I have also (as per the RSGB Rad Com Handbook and seen nowhere else)
>> rearranged the cables so that the braids are connected together and
>> grounded with the two inners providing the balanced line.
>>
>> On a small version of the DJ4VM quad as a test antenna, which is only
>> a couple of ohms on 20m, the K2 sounds dead. removing the braid and
>> using the feeder as a long wire brings the signals way up.
>>
>> My query is this. Is there an error in principle in using two pieces
>> of coax as a balanced line, which would imply the handbook is wrong or
>> am I missing something.
>>
>> I did wonder if the braid, used in this manner has a different
>> velocity factor to the inner and whether this could be the problem -
>> or am I trying to do the impossible?
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k6dgw

Re: OT- Using two coaxial cables as a balanced feeder

In reply to this post by Don Wilhelm-4

Don Wilhelm wrote:

> If your tuner has a limited range and cannot properly match the
> feedpoint impedance of the feedline, adding (or subtracting) a length of
> line to the feeder length can bring it into a range that your tuner can
> match. I would suggest adding or subtracting in 1/8th wavelength
> increments until you get it close to the desired range and then go to
> 1/16th wavelength additions or subtractions. The feedline acts like an
> impedance transformer when it is operated with an SWR other than 1.
>

Or, use N6BV's TLW [comes with the ARRL Antenna Book 20th ed] and just
calculate it. I modeled one of my low band antennas to get the feed
point impedance, and then used TLW to compute the open wire line length
to the balun. Worked great.

73,

Fred K6DGW
- Northern California Contest Club
- CU in the 2008 Cal QSO Party 4-5 Oct 08
- www.cqp.org
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