Telamon wrote:
In article ,
D Peter Maus wrote:

Telamon wrote:
In article ,
D Peter Maus wrote:

Telamon wrote:
In article ,
D Peter Maus wrote:

RHF wrote:
On Dec 30, 9:37 pm, D Peter Maus wrote:
Brian wrote:
I finally got a house out in the woods on five acres and I'm
going to set up a couple of pretty long wires and my Wellbrook
ALA-1530. I've pretty much decided on RG-6 for a couple of 150
ft. runs because the stuff's dirt cheap and I can pick it up at
the local Home Depot. Is the impedance mismatch here going to
be negligible, or should I just bite the bullet and go with
RG-8 or a similar 50 ohm coax?

-Brian
- Alone, your receiver won't care. I doubt that the Wellbrook would,
- either.
-
As a complex, however, you're going to have not one mismatch, -
but - two. One at the output of the Wellbrook loop, and one at -
the input - of - the antenna interface. This may result in
standing waves on the - transmission line which, in turn may
result in irregularities in - performance.
-
- With a reasonably well designed receiver, you'll likely not -
notice - any losses in practical listening. And unless you are
working at the - very limits of performance on signals very far
down in the noise and - doing A/B tests of one coax over another,
you'll not detect the - performance irregularities.
-

DPM,

Then it becomes a Practical Implementation and CBA type Problem.
CBA = Cost Benefit Analysis
http://en.wikipedia.org/wiki/Cost-benefit_analysis
http://en.wikipedia.org/wiki/Implementation

Cost of RG58/RG8 Coax Cable -versus- Cost of RG6 Quad-Shield
Coax Cable with a 75-to-50 Ohm Matching Transformer at each
end.

Starting with the Concept of a 9:1 Matching Transformer for General
Broad-Band Shortwave Radio (High Frequency 3~30 MHz) Use and
Application with 30 Turns (9-Side) and 10 Turns (1-Side) and the Same
Ferrite Core Material :

We now need a Matching Transformer to take the Antenna and Radios
SO-239 Jack/Plugs {BNC Connector Optional} -to- the 75 Ohm Coax
Cable with an F-Connector {BNC Connector Optional} .

The 50 Ohm 10 Turns (1-Side) should be understood.

Getting to the 75 Ohm (X-Side) should be simple Math :
75 / 50 = 1.5
Square Root of 1.5 = 1.225
Therefore the 75 Ohm (X-Side) would have 12 Turns.

A 75 Ohm to 50 Ohm [1.5:1] Matching Transformer would have
12 Turns (1.5-Side) and 10 Turns (1-Side) and the Same Ferrite
Core Material -as- 9:1 Matching Transformer for General Broad
Band Shortwave Radio (High Frequency 3~30 MHz) Use and
Application.

Anyone Else Have Any Ideas : On a Building Your Own {DIY}
a 75 Ohm to 50 Ohm [1.5:1] Matching Transformer ?

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
- - - - - - -


The Next Question Becomes : At What Cost ?

* Are the 75 Ohm to 50 Ohm [1.5:1] Matching Transformers
commercial available ? - a Ready Made Item ? Price ?

* Are the 75 Ohm to 50 Ohm [1.5:1] Matching Transformers a
Low Cost "Built-it-Yourself" Item ? - DIY Price ?

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
- - - - - - -

DPM - I go back to your first statement : "Alone, your receiver
won't care. I doubt that the Wellbrook would, either."


IMHO - For the vast majority of Shortwave Radio Listeners (SWLs)
Practical Implementation Says : Just Do It ! - Use the RG6 Coax
Cable -and- enjoy listening to your radios - iane ~ RHF
.


Which was precisely my point.
It's dumb to buy cable or anything else that is not matched to the task
unless there is some kind of over riding factor such as cost.

In your universe, and mine, yeah, you buy the best, brightest,
matched, optimized, or NFL approved.

Truth is, that in real world operations within the normal parameters
hobbyists achieve, there is likely no detectable difference between the
'correct' choice, and the most economical choice for transmission line
in this application. And given that the OP was concerned about cost, his
choice is a decent one.

And unless the OP is intending to operate his system at the limits of
performance, any losses incurred will not be relevant to his operation.


Would I do it that way? No. Would you do it that way? I'm thinking
not. But we're in a different position to make these decisions. And we
both have the hardware to not only notice the difference in
antenna/transmission line performance, but we operate closer to the
limits than the OP. For you and me, the choice of coax may well make a
detectable difference. In my case, it would also bother me to install a
line that I knew wasn't correct for the application.

There are others here with the same attention to detail. And still
others who will not hear or detect the differences.

You're running RX-340. The minutiae at the antenna can be heard.

It's not the case when the best receiver you've ever encountered is
Sat-800.
Well, yes performance is a critical parameter but I would not frame the
discussion around a high performance radio. The radio I own is not the
issue as ANY table top radio will be adversely affected in reception
performance. The performance issue that will affect ANY radio is due to
a recombination of forward and reflected waves that run the gamut of
reinforcement to cancelation. Where reinforcement or cancelation of
incoming signals will occur will depend on several factors and as a
consequence not easy to calculate. You may not ever hear a signal on
some frequency because that is where the cancelation occurs. The
cancelation has to happen somewhere in the frequency band. If you never
listen to that spot or spots then no problem but if it happens where you
want listen then tough luck.

Not every receiver will detect such cancellations at this operating
level, and with this magnitude of mismatch. Noise floor, location and
ambient noise will render some antenna losses irrelevant in practical
listening, and depening on frequency.

If you're not working at the limits of performance, small mismatches
in a receiving transmission line are trivial.

We're not talking about military grade monitoring of picowatt
signals, here.

This is not about scale. The signal can be very large and go to zero at
some frequencies. You are familiar with the Bose noise canceling
headphones? Same principle applies here.


It IS about scale. And no, it's not the same principle. Noise
cancelling headphones seek to remove all the noise. So, the antiwave is
of an amplitude approximately equal to the incoming noise. The losses in
a receiving antenna transmission line with a small mismatch don't
produce anywhere near that level of cancellation. A small mismatch
produces a small reflection. A small reflection only produces a partial
cancellation. A partial cancellation produces a lower incoming signal at
the partial cancellation frequency... the difference being recovered by
the sensitivity of the receiver and it's low noise floor. Unless the
hobbyist is operating at the limits of performance, that difference will
may never be noticed.





The general rule in a broad band system is to have components matched in
order to prevent these complex results of significant reflections.

I'll state again that if you already own the coax to try it, especially
if you only have interest in certain frequencies or have another radio
and antenna to check propagation. If it works where you want to listen
then that's just fine but buy the wrong coax? I would not recommend that.

Sometimes, the economics ARE the issue.

How much money can he save buying 300 feet of the 75 ohm coax over the
50 ohm? Maybe $30.



It IS his budget, after all. And for some, $30 can be a big
difference.

As I said, that level of investment may not matter to many of the
operators in the hobby. You and I wouldn't worry about it. But there are
some who have other priorities, and $30 is enough of a deal that it
warrants consideration.

The bottom line is that he's just getting into the hobby, again.
And the way the Wellbrook is designed, his mismatched cable run is on
the length between the loop and the preamp. Reflections in that line are
going to be small anyway. Cancellations are going to be partial at best,
and few in number, pursuant to length vs frequency. His choices will get
him up and running. Let him discover for himself how significant the
mismatch is.

Likely, he'll never detect it as you or I would.