[R-390] R-390A balanced input question

[Larry H]

Hi Jim and Craig,  Sorry, I got your names reversed on my previous post.

To all - It's important to note that the impedance on the C connector (J103) originally connected to P205, which is the hi impedance input is about 1 meg ohm.  This connection was designed to connect a whip antenna to it through a very short piece of coax.  The whip became part of the tuned circuit input to the rf amp and did not diminish the effectiveness of the secondary of T201-T206.  This allowed the Q of the antenna circuit to reduce unwanted images, adjacent stations and noise from entering the rf amp as much as possible for a whip antenna.  This also reduces IMD produced in the rf amp.  However, by connecting a low impedance antenna to this connection, the Q is severely reduced and the benefits of the tuned circuit are greatly removed.  If you want to take advantage of the filtering available in the antenna circuit for a low impedance antenna (ie any coax of some length (more than 20' from rx to antenna)) it's better to use the balanced input. 
 This can be done as explained elsewhere.  The impedance on rx's is not near as critical as tx's, so the small mismatch is not an issue.  The design of the R390-A's balanced input is good for impedances of 50 to 200 ohms.  Using an rf xformer to match it to 50 ohm coax is not productive.  I've tried many and the insertion loss is =/> a direct coax connection on very weak signals.

Where the impedance matching is important is when you are measuring signal to noise ratio using the balanced input.  Otherwise, it's not important to be right on.

Regards, Larry


On Sunday, September 21, 2014 5:12 PM, Craig Heaton <hamfish at efn.org> wrote:
 


Just an observation of someone that didn't have radio as an occupation. 

As Charles noted, an antenna tuner/matchbox is an exception to matching a
resonate antenna to 50 ohms. On my two stations using a tuner, the VSWR is
flat 1 to 1, and the antenna trim control is at 0 for max indication on the
carrier level meter. This is an indication peaking the RF section with
regard as to antenna type/plus tuner during RF alignment has advantages.  

Now, changing nothing but the KC knob, moving up or down in frequency, the
antenna trim will have to be tweaked to bring the carrier level back to a
peak. In my present use of a R-390/A paired with a transmitter, I'll take
the advantage of using the balanced antenna input & tuned primaries of
T201-T206. The antenna trim control will make up for things not quite at
optimum impedance at the antenna input.

YMMV
Craig 

-----Original Message-----
From: R-390 [mailto:r-390-bounces at mailman.qth.net] On Behalf Of Charles
Steinmetz
Sent: Sunday, September 21, 2014 4:11 PM
To: 390 list
Subject: Re: [R-390] R-390A balanced input question


>If you use the balanced input you can go through and trim up the 1st RF 
>stages. Make sure you use the same hook-up that you are going to run with.
>    *   *   *
>If you use a balun (2.25:1 or 2:5:1) then you need to do the alignment 
>through that connection. Make sure you set the ANT-TRIM knob to the center
>    *   *   *
>You are peaking performance. Usually I shoot for the middle of the band 
>that is associated with that RF stage. Since I use one of the 
>German-made baluns I do the  alignment from a 50 ohm unbalanced 
>connection from my signal generator

That's all well and good, but there are only a few, very narrow frequency
ranges between 500kHz and 32MHz at which the receiving antenna will be
anywhere near 50 ohms.  So in the end, anything you can do with a 50 ohm
generator isn't going to be very helpful because using a 50 ohm generator
violates the first principle above, "Make sure you use the same hook-up that
you are going to run with."

Instead, people with random antennas should ignore the balanced input, feed
the unbalanced input, and ignore all the whining they've seen here and
elsewhere about the unbalanced input "bypassing important tuned circuits."
All you're missing are the tuned primaries of T201-206 (depending on the
band selected), and if you aren't matched to them (and the paragraph above
explains why you won't be), you're much better off without them.  As a side
benefit, the front end noise of the radio will usually drop significantly
(how much depends on your particular antenna).

The only exception would be if you use an antenna tuner ("matchbox") to
match a resonant antenna to 50 ohms, or if you cut an antenna specifically
to be 50 ohms at some important frequency -- but this can only be true for
very narrow frequency bands, because antenna resonances are very narrow.

Best regards,

Charles



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