Red Wave Radio

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

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

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

bart   wb6hqk

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

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

73, Cecil, W6RCA, OOTC

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

73 Danny

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

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

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

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

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?

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

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

I need to digest this a bit.

Danny, K6MHE

73, Cecil, W6RCA, OOTC

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

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

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

73

Danny, K6MHE