More on "Coax as Balanced Feedline"

More on "Coax as Balanced Feedline"

Post by Paul Racin » Wed, 19 Mar 1997 04:00:00

Thanks for your comments, Dave.  
---

Paul, I have been running some tests with twin coax feed as a balanced line
and when I compare it with a dipole fed with 450 ohm line no one can tell
the difference.  have you been able to determine the what losses may occur
with it?  I am using it to feed a 160 m dipole as a multiband center fed,
and it is tuned with the balanced tunner from QST a few years back.  I
would appreciate any feed back you have.
----

Dave, I have not dtermined the losses which may occur using this system.  I
would guess that the losses are higher than that of open balanced line, but
lower than that of a single unblanced coax feedline......I know from my
experience, just as you have, that there was no real difference  when
compared to open 450 ohm line.  The balnced  coax feedline was more
convenient because I didn't have to be too concerned about where exactly I
ran the feedline when in the house (near copper heating pipes, wires,
etc.).

When I was using the system (I am using a Carolina Windom antenna, now, fed
with coax) a year ago, I wanted to keep my system as balanced as possible.
So, what I decided to abandon the voltage 4:1 balun on my MFJ tuner, and I
did not have a balanced tuner.  So, what I did was isolate my tuner from
ground, used the chassis of the tuner as on end of the balanced line, and
used the long wire connection as the other end of the balanced line...then,
to preserve the balanced system, I installed an RF ***(Isolator from
RADIO WORKS) between the tuner and the radio...

THIS WORKED GREAT, and I had NO rf radiating in my house off the feedline,
no RF on the chassis of the radio, NOTHING.  I had a truely balanced
system!  Comparing this to using just the 4:1 (voltage) balun built into
the tuner, there was no contest!!!  This was the first time during my ham
career that I had no TVI problem.

Thanks for your reply, Dave....

---Paul, AA8RC

 
 
 

More on "Coax as Balanced Feedline"

Post by Dan K6M » Wed, 19 Mar 1997 04:00:00



>Thanks for your comments, Dave.  
>---

>Paul, I have been running some tests with twin coax feed as a balanced line
>and when I compare it with a dipole fed with 450 ohm line no one can tell
>the difference.  have you been able to determine the what losses may occur
>with it?  I am using it to feed a 160 m dipole as a multiband center fed,
>and it is tuned with the balanced tunner from QST a few years back.  I
>would appreciate any feed back you have.
>----

>Dave, I have not dtermined the losses which may occur using this system.  I
>would guess that the losses are higher than that of open balanced line, but
>lower than that of a single unblanced coax feedline......I know from my
>experience, just as you have, that there was no real difference  when
>compared to open 450 ohm line.  The balnced  coax feedline was more
>convenient because I didn't have to be too concerned about where exactly I
>ran the feedline when in the house (near copper heating pipes, wires,
>etc.).

[snip]
>---Paul, AA8RC

Hi Paul,

The losses using twin coax as you described would be the same as using
single coax. At first I thought the parallel arrangement would have
less losses too, however, when I check it out with some people
knowledgeable on the subject I found out that the loss is the same.
Examination will reveal that each length of coax has ? the current (?
the loss) of a single length of coax, however, there are two lengths.
The result is that the loss remains the same.

What I have done in my installation is to use 450 ohm ladder line
running from the antenna to the outside wall of the shack. At that
point I have connected the ladder line to a short run (3 feet in my
case) of parallel RG-213 coax. The parallel coax line is then brought
through the wall of the sack to the antenna tuner. I feel this
arrangement offers me good low loss feed line system plus it costs
less to install than running the parallel coax all the way to the
antenna.

I agree with you that most stations would probably not be able to note
any real difference in my signal strength using this method over
running parallel coax all the way to the antenna but, I can have that
warm fuzzy feeling that it is working better.  Hi..

73,

Danny, K6MHE

 
 
 

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Post by Bart Rowle » Mon, 31 Mar 1997 04:00:00


>Hi Paul,

>The losses using twin coax as you described would be the same as using
>single coax. At first I thought the parallel arrangement would have
>less losses too, however, when I check it out with some people
>knowledgeable on the subject I found out that the loss is the same.
>Examination will reveal that each length of coax has ? the current (?
>the loss) of a single length of coax, however, there are two lengths.
>The result is that the loss remains the same.

When operating without standing waves, the loss of the twin coax feeder
will be the same as a single unbalanced piece of the same coax.  Assuming
50 ohm coaxial cable is used in both cases, the twin coax configuration
would need to be terminated in a 100 ohm resistive load to meet the standing
wave criteria.  If the load impedance were 100 ohms resistive, the twin coax
would have lower loss. If the load were 50 ohms resistive, the single coax
feeder would be lower loss.  It gets more interesting if the load becomes
reactive.  The VSWR will be lower on the higher impedance (twin coax) feeder
and the losses will be lower.  If one is insistent on using coax, it may make
sense to use the twin coax feeder with full wave loops but probably not center
fed half wave dipoles except perhaps on 75/80 meters.  

BTW, one half the current would result in one quarter the losses.

bart   wb6hqk


 
 
 

More on "Coax as Balanced Feedline"

Post by Dan K6M » Wed, 02 Apr 1997 04:00:00

Quote:

>When operating without standing waves, the loss of the twin coax feeder
>will be the same as a single unbalanced piece of the same coax.  Assuming
>50 ohm coaxial cable is used in both cases, the twin coax configuration
>would need to be terminated in a 100 ohm resistive load to meet the standing
>wave criteria.  If the load impedance were 100 ohms resistive, the twin coax
>would have lower loss. If the load were 50 ohms resistive, the single coax
>feeder would be lower loss.  It gets more interesting if the load becomes
>reactive.  The VSWR will be lower on the higher impedance (twin coax) feeder
>and the losses will be lower.  If one is insistent on using coax, it may make
>sense to use the twin coax feeder with full wave loops but probably not center
>fed half wave dipoles except perhaps on 75/80 meters.  

>BTW, one half the current would result in one quarter the losses.

>bart   wb6hqk

Hi Bart,

OOPS, should have said half the power. Thanks for pointing this out.

Could you elaborate on the 50 ohm situation you mentioned in your
post? I understood it to be the same regardless of the SWR.

73
Danny, K6MHE

 
 
 

More on "Coax as Balanced Feedline"

Post by Cecil Moor » Wed, 02 Apr 1997 04:00:00


> Could you elaborate on the 50 ohm situation you mentioned in your
> post? I understood it to be the same regardless of the SWR.

Hi Danny, the loss is the same regardless of the SWR *if and only
if the SWRs are equal*. When feeding a 50 ohm load with 50 ohm coax
the SWR is 1:1. When feeding a 50 ohm load with a balanced run of
50 ohm coax the SWR is 2:1, not equal SWRs so not equal losses.

73, Cecil, W6RCA, OOTC

 
 
 

More on "Coax as Balanced Feedline"

Post by Dan K6M » Sat, 05 Apr 1997 04:00:00

On Tue, 01 Apr 1997 09:09:03 -0700, Cecil Moore



[snip]
>Hi Danny, the loss is the same regardless of the SWR *if and only
>if the SWRs are equal*. When feeding a 50 ohm load with 50 ohm coax
>the SWR is 1:1. When feeding a 50 ohm load with a balanced run of
>50 ohm coax the SWR is 2:1, not equal SWRs so not equal losses.

>73, Cecil, W6RCA, OOTC

I sorry Cecil, but, I guess I'm lost some where. Why is it not equal?

73 Danny

 
 
 

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Post by Cecil Moor » Sat, 05 Apr 1997 04:00:00


> I sorry Cecil, but, I guess I'm lost some where. Why is it not equal?

Hi Danny, I infer from your question that you don't know that if two
side-by-side runs of coax are used in a balanced configuration, that
the characteristic impedance is twice the characteristic impedance of
a single run of that same coax. So the Z0 of a side-by-side run of
50 ohm coax is 100 ohms.

If the side-by-side run sees 50 ohms the SWR is 100/50 = 2:1

If the side-by-side run sees 100 ohms the SWR is 100/100 = 1:1

If the single run sees 50 ohms the SWR is 50/50 = 1:1

If the single run sees 100 ohms the SWR is 100/50 = 2:1

Since the losses are the same only if the SWR is the same:

The losses are the same if the side-by-side run sees 100 ohms and
the single run sees 50 ohms.

The losses are the same if the side-by-side run sees 50 ohms and
the single run sees 100 ohms.

Note the losses are the same for both feedlines only if the load is
70.7 ohms resulting in identical SWRs of 1.414.

73, Cecil, W6RCA, OOTC

 
 
 

More on "Coax as Balanced Feedline"

Post by Dan K6M » Sun, 06 Apr 1997 04:00:00

On Fri, 04 Apr 1997 10:46:56 -0700, Cecil Moore



>> I sorry Cecil, but, I guess I'm lost some where. Why is it not equal?

>Hi Danny, I infer from your question that you don't know that if two
>side-by-side runs of coax are used in a balanced configuration, that
>the characteristic impedance is twice the characteristic impedance of
>a single run of that same coax. So the Z0 of a side-by-side run of
>50 ohm coax is 100 ohms.

Yes I know and think I understan that.

Quote:

>If the side-by-side run sees 50 ohms the SWR is 100/50 = 2:1

>If the side-by-side run sees 100 ohms the SWR is 100/100 = 1:1

>If the single run sees 50 ohms the SWR is 50/50 = 1:1

>If the single run sees 100 ohms the SWR is 100/50 = 2:1

>Since the losses are the same only if the SWR is the same:

>The losses are the same if the side-by-side run sees 100 ohms and
>the single run sees 50 ohms.

>The losses are the same if the side-by-side run sees 50 ohms and
>the single run sees 100 ohms.

>Note the losses are the same for both feedlines only if the load is
>70.7 ohms resulting in identical SWRs of 1.414.

>73, Cecil, W6RCA, OOTC

Cecil, let me ask the question a little different way.

Lets say that I am feeding a dipole which not physical resonate and
(we will say for my question) has a feed point impedance 500 ohms.

Condition one feed with single RG-11 (75 ohm coax). What will the
additional loss due to swr on the feedline be with a transmitter
output power of 100 watts? What would be to total transmission line
losses be?

Condition two feed with two parallel runs of the same coax. What will
the additional loss due to swr on the feedline be with the same
transmitter output power? What would be to total transmission line
losses be?

For these questions I don't want to go into common mode current
question - just assume that for what ever reason that equal current is
flowing between the center conductor and the inside shield of the
single run of coax.

73 Danny, K6MHE

 
 
 

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Post by Cecil Moor » Sun, 06 Apr 1997 04:00:00


> Lets say that I am feeding a dipole which not physical resonate and
> (we will say for my question) has a feed point impedance 500 ohms.

> Condition one feed with single RG-11 (75 ohm coax). What will the
> additional loss due to swr on the feedline be with a transmitter
> output power of 100 watts? What would be to total transmission line
> losses be?

Hi Danny, the SWR would be 500/75=6.67. Matched line loss for 100ft
at 10 MHz is about 0.66dB. From a chart in the ARRL Antenna Book, the
total loss would be 1.8dB (34%) or 1.14dB due to SWR. Matched line
loss means a load equal to Z0=75 ohms.

Quote:> Condition two feed with two parallel runs of the same coax. What will
> the additional loss due to swr on the feedline be with the same
> transmitter output power? What would be to total transmission line
> losses be?

The SWR in this case would be lower and therefore the losses will be
lower. SWR would be 500/150=3.33. The chart says the total loss in
this case would be about 1.1dB (22%) or 0.44dB due to SWR. Matched
line loss means a load equal to Z0=150 ohms.

You are about 0.7dB (15%) better off using the side-by-side coax. But
for low impedances around 50 ohms, you would be worse off.

73, Cecil, W6RCA, OOTC

 
 
 

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Post by Dan K6M » Mon, 07 Apr 1997 05:00:00

Quote:

>The SWR in this case would be lower and therefore the losses will be
>lower. SWR would be 500/150=3.33. The chart says the total loss in
>this case would be about 1.1dB (22%) or 0.44dB due to SWR. Matched
>line loss means a load equal to Z0=150 ohms.

>You are about 0.7dB (15%) better off using the side-by-side coax. But
>for low impedances around 50 ohms, you would be worse off.

>73, Cecil, W6RCA, OOTC

Thanks Cecil,

I need to digest this a bit.

Danny, K6MHE

 
 
 

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Post by Cecil A. Moor » Mon, 07 Apr 1997 05:00:00


> Would you say that if the SWR is the same on
> both feedlines (irrespective of the fact that the load impedances must
> be different to achieve that situation) the losses will be the same?
> Assuming that both lines would be using the same coax and the same
> lengths.

Hi Danny, actually there is a load impedance that will cause the same
SWR for 75 ohm coax and parallel 150 ohm, side-by-side 75 ohm coax.
A 106.066 ohm load will cause an SWR of 1.414 on either feed system so,
yes, the losses will be the same if the SWR is the same. If the SWR is
not the same, the losses will be highest in the system with the
highest SWR assuming the same coax and the same lengths.

73, Cecil, W6RCA, OOTC

 
 
 

More on "Coax as Balanced Feedline"

Post by Dan K6M » Tue, 08 Apr 1997 04:00:00

On Sat, 05 Apr 1997 22:24:48 -0700, Cecil Moore



>> Lets say that I am feeding a dipole which not physical resonate and
>> (we will say for my question) has a feed point impedance 500 ohms.

>> Condition one feed with single RG-11 (75 ohm coax). What will the
>> additional loss due to swr on the feedline be with a transmitter
>> output power of 100 watts? What would be to total transmission line
>> losses be?

>Hi Danny, the SWR would be 500/75=6.67. Matched line loss for 100ft
>at 10 MHz is about 0.66dB. From a chart in the ARRL Antenna Book, the
>total loss would be 1.8dB (34%) or 1.14dB due to SWR. Matched line
>loss means a load equal to Z0=75 ohms.

>> Condition two feed with two parallel runs of the same coax. What will
>> the additional loss due to swr on the feedline be with the same
>> transmitter output power? What would be to total transmission line
>> losses be?

>The SWR in this case would be lower and therefore the losses will be
>lower. SWR would be 500/150=3.33. The chart says the total loss in
>this case would be about 1.1dB (22%) or 0.44dB due to SWR. Matched
>line loss means a load equal to Z0=150 ohms.

>You are about 0.7dB (15%) better off using the side-by-side coax. But
>for low impedances around 50 ohms, you would be worse off.

>73, Cecil, W6RCA, OOTC

Cecil,

An additional question. Would you say that if the SWR is the same on
both feedlines (irrespective of the fact that the load impedances must
be different to achieve that situation) the losses will be the same?
Assuming that both lines would be using the same coax and the same
lengths.  

Danny, K6MHE

 
 
 

More on "Coax as Balanced Feedline"

Post by Dan K6M » Tue, 08 Apr 1997 04:00:00

On Sun, 06 Apr 1997 23:54:15 -0700, "Cecil A. Moore"



>> Would you say that if the SWR is the same on
>> both feedlines (irrespective of the fact that the load impedances must
>> be different to achieve that situation) the losses will be the same?
>> Assuming that both lines would be using the same coax and the same
>> lengths.

>Hi Danny, actually there is a load impedance that will cause the same
>SWR for 75 ohm coax and parallel 150 ohm, side-by-side 75 ohm coax.
>A 106.066 ohm load will cause an SWR of 1.414 on either feed system so,
>yes, the losses will be the same if the SWR is the same. If the SWR is
>not the same, the losses will be highest in the system with the
>highest SWR assuming the same coax and the same lengths.

>73, Cecil, W6RCA, OOTC

Thanks. That answers my questions.

73

Danny, K6MHE