[R-390] Balanced Antenna Connector

Bob Camp kb8tq at n1k.org
Wed Oct 9 20:28:24 EDT 2024 

Hi

Again, all of this *assumes* a number of things. One of may is that you do 
have 1 uV sensitivity. If the radio is “broke” in some way and you are at 10 uV or
100 uV then that needs to be fixed. 

With something like a Grundig RF-50U, you may well be using a very different 
antenna than your R-390. Unless the antenna is same / same between two radios,
comparing them is not really fair.

Bob

> On Oct 9, 2024, at 8:12 PM, Barry Scott <72volkswagon at gmail.com> wrote:
> 
> One of the reasons I'm looking at the balanced input is how it might
> help with weak signals and have I done myself a disservice by aligning
> the radio using the unbalanced input.  I think that downstream from
> T20*, it shouldn't matter but that may be incorrect.  If correct, then
> I will do the alignment again using the balanced input.
> 
> What got me started is that there's a radio station about 100 miles
> north of my location (WSM outside of Nashville) that I sometimes like
> to tune in; however, on this R-390/URR, it has been quite weak.  I was
> blaming propagation but then I tried tuning that station in with my
> vintage Grundig RF-80U and I was surprised that I could hear it as
> good if not a little better than on my R-390/URR.  That was
> disappointing and makes me think I have a sensitivity issue - at least
> on the 0.5-1.0MC band.
> 
> Thanks,
> Barry - N4BUQ
> 
> On Wed, Oct 9, 2024 at 3:10 PM Bob Camp <kb8tq at n1k.org> wrote:
>> 
>> Hi
>> 
>> Does any of this matter in a real setup?
>> 
>> If you make some assumptions to get the math going (yes, change the assumptions and the details change …):
>> 
>> 1) You are running an 8 KHz “IF bandwidth”.
>> 
>> 2) Your 8 KHz filter has a 10 KHz “noise bandwidth” (noise bandwidth will always be larger by some amount … ).
>> 
>> 3) Your antenna input is looking at a 50 ohm resistor in a nice warm room. The radio does not load things down. The input is at 50 ohms.
>> 
>> (like I said … assumptions …).
>> 
>> You get roughly -133 dbm of RF power at the antenna input in that bandwidth simply from thermal noise. More or less, that’s 0.05 uv (ALL of these numbers rounded a bit to keep things simple).
>> 
>> You now go to measure sensitivity and are looking for a 10 db signal to noise ratio.
>> 
>> 1) That’s about a 3:1 ratio
>> 
>> 2) If thermal noise was all that mattered we would have a 0 db noise figure front end
>> 
>> 3) With this ideal front end, our signal would be at 3 x 0.05 uV. That gets us to 0.15 uV.
>> 
>> If our radio comes in at 1 uV sensitivity, it’s noise figure is 20 log ( 1 / 0.15 ). That would be 16 db. Measure something else and you have a different noise figure. Change that noise bandwidth or open a window to cool down that warm room, it changes as well.
>> 
>> There are lots and lots of folks who have looked at the RF floor in various locations and frequencies. As more and more folks generate RFI those numbers just get worse and worse.
>> 
>> Next up, what sort of antenna are you using?
>> 
>> Turns out that longer antennas likely give you more signal. If you have a 6” piece of wire, it’s not going to do quite as well as a 60’ piece of wire. (all other things being equal). Most of us are not going to be using a 6” piece of wire *and* expecting to get super duper performance.
>> 
>> With a typical antenna and at most of the frequencies an R390(a) covers, the noise on the antenna is *way* more than that 16 db noise figure. Have a mismatch loss? It’s still way more. Have a weird input connection? still way more.
>> 
>> One of *many* articles on HF noise floor:
>> 
>> http://rsgb.org/main/files/2017/12/221216-Noise-leaflet-issue-2.pdf
>> 
>> Ok, you don’t trust the math. Really simple way to test this:
>> 
>> Fire up the radio with the input shorted (or hooked to a dummy load). See what the output is (ideally with AGC turned off and the RF gain turned all the way up).
>> 
>> Hook up the antenna.
>> 
>> Does the noise coming out of the speaker go up? (it should ….)
>> 
>> Does it go up by more than 3 db? (I’d bet it does).
>> 
>> If so, the antenna “noise floor” is what rules. No need or value in getting any crazier about this. What you have is good enough.
>> 
>> Ok, so the noise goes up a lot. Isn’t that the best thing to have happen?
>> 
>> Well, not so much. Your poor radio also has to deal with issues like overload and other problems from strong signals at the antenna. In some cases a few db increase in those signals means quite a few db increase in the amount of trouble they cause. Adding gain you don’t need is simply creating issues you could avoid.
>> 
>> Fun !!!
>> 
>> Bob
>> 
>> 
>> 
>>> On Oct 9, 2024, at 3:09 PM, Barry <n4buq at knology.net> wrote:
>>> 
>>> That may very well be what I'm thinking of.  This is what I remember doing for one of my R-390As.  I think winding a toroidal transformer may work better.  I used to have a box that connected to the balanced jack and had a BNC connector for the unbalanced coax line to the antenna.  I think it wasn't an exact match - something like 65 ohms - but it worked.  I don't think I'd be able to use it, though, with this jack.  I'm still unsure what it is.
>>> 
>>> Thanks,
>>> Barry - N4BUQ
>>> 
>>>> I am aware of a cap/resistor combo Chuck recommended for outputting the diode
>>>> load to an outboard preamp/amp combination. Perhaps that was what you’re
>>>> thinking of?
>>>> 
>>>> 
>>>> 
>>>> Dan - WB4GRA
>>>> 
>>>> 
>>>> 
>>>> 
>>>> Sent from my iPhone
>>>> 
>>>>> On Oct 9, 2024, at 2:25 PM, Barry Scott <72volkswagon at gmail.com> wrote:
>>>>> 
>>>>> What is the correct designation for the two-pin antenna connector
>>>>> (balanced) in the R-390/URR (likely the same for the R-390A/URR but I
>>>>> want to be specific just in case).
>>>>> 
>>>>> Also, I think Chuck Rippel had a connection contraption that used a
>>>>> resistor (or two) so that 50-ohm unbalanced would match just a bit
>>>>> better using the balanced connection.  He had a website that depicted
>>>>> that but I don't know if that still exists.  Anyone know?
>>>>> 
>>>>> Thanks,
>>>>> Barry - N4BUQ
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