[R-390] R-390 Balanced Antenna Input
Bob Camp
ham at cq.nu
Sun Jun 27 10:24:24 EDT 2004
Hi
The performance actually is a bit harder to evaluate than simply
switching between the two inputs.
To get the unbalanced input set up right you need to align the radio
out of a 50 ohm generator hooked up unbalanced to the radio. In this
configuration the balance tuning adjustment does not do what it is
supposed to do. In addition the coils peak at a different point for a
low impedance generator than they do for a high impedance generator.
Given the way the circuit is set up that's exactly what you would
expect them to do.
On the balanced input you need to feed the input with a proper
impedance matching network. There are examples in most of the manuals.
Usually they are just a bunch of resistors rather than transformer
based. Once you get the radio set up from the match network there is
another step that does the balancing process. Since the matching
networks are usually unbalanced to ground this isn't quite as easy as
it might be.
In both cases the result still won't be a 50 or 125 ohm input impedance
on the radio. The result will be the best performance you can get from
a 50 ohm or 125 ohm source. The difference is a bit subtle but the
resulting impedance may be a half or 2X what you would expect it to be.
If you terminate a transmission line in it's characteristic impedance
then it does not much mater what length you use. If you go to an
impedance that is significantly different than the characteristic
impedance you want to use half wave multiples lengths of line. This
works fine in a single frequency receiving setup. It gets a bit complex
otherwise.
None of this came as a big surprise to the guys who designed the radio.
That's why you have a nice big antenna trim control on the front panel.
Since the antenna may be both reactive and at an odd impedance you can
only do so much with a single control. A full blown antenna matcher
will beat a single control part of the time. What's amazing is that
modern radios have forgotten about all this. We have all kinds of weird
knobs on a modern radio but nothing much to make it tune up the antenna
on receive.
There does not seem to be any compelling argument to go to 125 ohms
balanced other than to reduce the effect of noise in the shack.
Matching a normal vertical or dipole to 125 ohms isn't terribly easy.
Of course once you go off of the design frequency a vertical will do a
bit better into a higher impedance. Getting this done with a chunk of
coax between the radio and the antenna messes the whole process up to
the point that the net result is hard to predict.
These days we run the 390's without upper and lower covers in place. A
lot of radios have also lost the cover that goes over the RF deck. This
is a great idea when it comes to cooling the radio and keeping it
working for a long time. It's not as good idea in terms of shielding
the radio from crud in the shack. The net result is that even though
the balanced 125 ohms may help reduce the crud it's won't do as good a
job as it might.
Any time you hook an antenna to a radio and the noise out of the radio
goes up the antenna and not the radio is determining what is going on
at the front end. As long as this is true you have more sensitivity
than you need. You can still have two radios that do this and they will
still hear different things. If they do it's not because one has a more
sensitive front end than that other one. It's because the filters,
detectors, or audio properties are different. The difference may even
be a result of front end overload or distortion effects in the two
radios.
One nice thing about the R-390 radios is that they give you pretty darn
good sensitivity *and* front end selectivity. This is not always the
case with more modern radios. Often they make some pretty major
compromises in either the sensitivity or selectivity department. Of
course many of them were designed for a very different mission than an
R390. It's a bit tough to auto tune a R390 over telephone line that's a
couple hundred miles long ....
So how does any of this relate to 50 ohms versus 125 ohms you ask .....
The front end of the radio will have different selectivity
characteristics when it is driven off of 50 ohms and 125 ohms. It's not
quite as simple as larger resistance = higher Q = more selectivity. In
fact the truth may be exactly the opposite. You could do some analysis
to see what's going on but that's more work than I'm up to on a Sunday
morning. Since you don't really know what the impedance of the antenna
is I'm not sure how useful the result would be.
People have constructed 50 ohm to 125 ohm transformer match networks to
go from an unbalanced 50 ohm line to a 125 ohm balanced input. There
have not been any reports of significantly improved on the air
performance when running this kind of setup. The only observation that
I have made is that the 125 ohm match seems to keep the trimmer caps on
the 30 MHz end of the RF coils better in range. When you set it up with
a 50 ohm input these caps often wind up at one end of their range.
Both configurations are representative of how the radios were used
when in military service. f there had been a significant problem with a
50 ohm shipboard setup they certainly had the skills to put a matcher
between the antenna and the radio. Bottom line seems to be that they
never bothered. They did do a routine impedance sweep of the antennas
so it was no secret what the impedances involved were.
I have no doubt that a setup of multiple big rhombics with combiner
transformers and balanced line was a better setup than a 35 foot whip
on the deck of a destroyer. That probably had a bit more to do with the
antennas than it did with 50 ohms versus 125 ohms .... Of course if
anyone would like to forward me the two setups for evaluation I'll be
more than wiling to post the results.
Bottom line - if you happen to have a 125 ohm balanced antenna farm
then by all means run 125 ohm balanced it works just fine. For the rest
of us who have 50 or 75 ohm antenna farms there is no big reason to
worry about it.
Once BPL comes to all our neighborhoods this will be a moot point
anyway. Maybe we can use our radios as planters ...
Take Care!
Bob Camp
KB8TQ
On Jun 27, 2004, at 3:48 AM, Glen Galati wrote:
> Hello Walter & Group,
> Was this WE754 CABLE SHIELDED AND WHAT IS THE IMPEDANCE?
> I have a balanced coaxial cable jumper that is RG-22B/U and is
> terminated
> with UG-421 Amphenol TWIN-AX male connectors. The Impedance is 95
> Ohms @ 16
> pfd/ft. This coax was made by I.T.&T. Federal Cable. I know that the
> Velocity of Propagation increases with Impedance increases. Most of
> our
> RADAR equipment used RG-62 coax when outside the waveguide
> environment. In
> all my years in the Air Force I never saw balanced antenna lines used.
> I may
> have missed some important thread explanation on the subject of the
> OR-390
> Balanced Input, so I'm not up to speed on which produces the best
> results.
> QUESTION: On a side by side comparison with say two dipoles cut to the
> same
> frequency WITH one using balanced feed-line and the other coax, which
> would
> win out? There is an interesting coax by BELDEN 9857 (MIL-C-17F)
> RG-63 that
> is 125 ohms, 84% V.F. It has two shields (1-copper & 1 tinned copper)
> very
> low loss 1.5 db/100 ft. The center conductor is solid 22 AWG and is a
> copper
> clad steel. There are also two BELDEN 124 Ohm TWIN-AX #9271 (66%VF) &
> 9860
> (78% VF). I haven't seen this coax and anyone with an interest can
> surely
> get all the specs from the Belden website.
>
> 73,
> Glen Galati, KA7BOJ
> TACOMA, WA
> eldim at worldnet.att.net
>
>
> ----- Original Message -----
> From: "Walter Schulz" <k3oqf at localnet.com>
> To: <r-390 at mailman.qth.net>
> Sent: Saturday, June 26, 2004 9:34 AM
> Subject: [R-390] R-390 Balanced Antenna Input
>
>
> Regarding the balanced Antenna Input. Western Electric use to make
> WE754
> Cable which was double shielded and used for the transmission of
> video. DB
> loss per 100 feet at 148 Mhz was less than 1DB.
>
> There were two leads, white was the tip, and blue was the ring side.
> The
> connectors were made by AMP connectors, and would assume they are still
> available.
>
> Using for WE754 for ham radio or unbalance, simply twist the white and
> blue
> wires together and solder. The input impedance becomes approximately 52
> ohms.
>
> I thought this might be helpful to all who own R-390A receivers.
>
> Walter
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