Feedline impedance query
 

If I join 2 lengths of 93 ohm coax to give 46.5 ohm impedance will the db
loss on the line be greater,less or the same as a single lengthof the coax?
ZL2DG

Feedline impedance query
 

On Sun, 18 Jun 2006 16:34:19 +1200, "David & Margaret McBeth"
wrote:

If I join 2 lengths of 93 ohm coax to give 46.5 ohm impedance will the db
loss on the line be greater,less or the same as a single lengthof the coax?
ZL2DG


Presumably by "join 2 lengths" you mean to connect two lengths in
parallel.

If the loss factor L is the loss in one length of the cable, won't the
loss in two parallel lengths be Plost=Pin/2*L+Pin/2*L, so Plost/Pin=L?

Owen
--

Feedline impedance query
 

On Sun, 18 Jun 2006 05:10:58 GMT, Owen Duffy wrote:

On Sun, 18 Jun 2006 16:34:19 +1200, "David & Margaret McBeth"
wrote:

If I join 2 lengths of 93 ohm coax to give 46.5 ohm impedance will the db
loss on the line be greater,less or the same as a single lengthof the coax?
ZL2DG


Presumably by "join 2 lengths" you mean to connect two lengths in
parallel.

If the loss factor L is the loss in one length of the cable, won't the
loss in two parallel lengths be Plost=Pin/2*L+Pin/2*L, so Plost/Pin=L?


Hi Owen,

Hmm, each time I look at these equations, something is missing, but I
don't know what (probably because it is missing).

Simply put, two lines in parallel lose as much power as one line.
Nothing to be gained in that regard.

73's
Richard Clark, KB7QHC

Feedline impedance query
 

On Sat, 17 Jun 2006 22:16:58 -0700, Richard Clark
wrote:


If the loss factor L is the loss in one length of the cable, won't the
loss in two parallel lengths be Plost=Pin/2*L+Pin/2*L, so Plost/Pin=L?


Hi Owen,

Hmm, each time I look at these equations, something is missing, but I
don't know what (probably because it is missing).

Devilishly clever technique that. I can't really help, I didn't see it
even after I pressed the send button!


Simply put, two lines in parallel lose as much power as one line.

Of course, we both assumed the lines operates with VSWR=1.

Nothing to be gained in that regard.

Pardon the pun!

Owen
--

Feedline impedance query
 

On Sun, 18 Jun 2006 05:33:00 GMT, Owen Duffy wrote:

Simply put, two lines in parallel lose as much power as one line.

Of course, we both assumed the lines operates with VSWR=1.

Hi Owen,

You assumed twice. SWR will only mean more loss - the same loss for
one or two lines either way.

73's
Richard Clark, KB7QHC

Feedline impedance query
 

David & Margaret McBeth wrote:
If I join 2 lengths of 93 ohm coax to give 46.5 ohm impedance will the db
loss on the line be greater,less or the same as a single lengthof the coax?

You have not supplied enough information. What is the
load impedance in each case?
--
73, Cecil http://www.qsl.net/w5dxp

Feedline impedance query
 

Richard Clark wrote:
Simply put, two lines in parallel lose as much power as one line.
Nothing to be gained in that regard.

Doesn't it depend on the value of the load impedance,
i.e. upon the SWR?
--
73, Cecil http://www.qsl.net/w5dxp

Feedline impedance query
 

It is perfectly obvious, at DC and low frequencies, the resistances of
the two inner conductors are in parallel with each other. So with a
fixed voltage at one end there will be a greater power dissipated in a
load resistance at the receiving end.

Outer conductor resistance has a secondary effect.

By connecting two cables in parallel the resulting attenuation will be
smaller. But as Cecil implies, for exact calculations, cable
impedance, cable length, generator and load resistances, and SWR enter
the argument.

In general, it is a complete waste of time, trouble and cost to
connect coaxial cables in parallel merely to obtain a different Zo
impedance.

If a reduction in line loss is the objective, then the most economic
and effective procedure is to use thicker wires in the transmission
line and let line impedance look after itself.
----
Reg. G4FGQ

Feedline impedance query
 

Reg Edwards wrote:
In general, it is a complete waste of time, trouble and cost to
connect coaxial cables in parallel merely to obtain a different Zo
impedance.

Sounds like someone might have some free 93 ohm coax
and be wanting to use it on a dipole.
--
73, Cecil http://www.qsl.net/w5dxp

Feedline impedance query
 

David & Margaret McBeth wrote:
If I join 2 lengths of 93 ohm coax to give 46.5 ohm impedance will the db
loss on the line be greater,less or the same as a single lengthof the coax?
ZL2DG

Assuming the cables are matched in each case, loss is the same.

Think about it this way. You have half the power through each cable.
This means whatever loss you have, each cable now has half the power
loss. When they are summed back together, losses are now the same.

Say you had one cable with 100 watts, and power loss was 1 watt. Now
you use two cables and 50 is 50 watts in each and loss is 1/2 watt. You
sum them together and you get 1 watt loss, and now have 99 watts.

That's the matched case.

In the mismatched case in limited conditions, what Reg says will work.
For example at VHF and lower frequencies most cables are dominated by
conductor losses. At cable less than 1/4 wl long will have decreased
loss when it has some SWR in a direction that decreases current. By
paralleling two cables that are LESS than 1/4 wl long, as long as
conductor resistance losses dominate, you can find situations where
losses decrease if the mismatch is in a direction that decreases line
current.

If the lines are long increased SWR always causes increased loss. You'd
have to be careful to avoid making a change that causes each line to
have increased loss from standing waves, because if that happened line
loss would increase.

Since 93 ohm line has terrible loss, I'd avoid it. A good line would be
a short length of parallel 75 ohm line, but then who needs a 37.5 ohm
line? I did that once on a 160 vertical, but the .05dB power I saved
wasn't the reason. The real reason was so the 35 ohm antenna feedpoint
stayed where it was instead of being transformed by the line.

73 Tom