SDS100/SDS200: Extremely Deaf Below 85mhz

[dazza0768]

why is the sds100 and sds200 extremely deaf below 85mhz. In New Zealand our fire service channels are around 75mhz on VHF low.
My BCD436HP and UBCD325P2 pick it up no issues but the USDS100 is completely deaf. Obviously different receiver technology has a part to play.
It is good as gold when plugged into my discone via coax but any hand held antenna doesn't work good at all. Its also good as gold when the power cable is plugged in, even if its just the USB cable without power.
My friend has an SDS200 and has the same issue. Its a widespread issue among all sds models.
Uniden could have done a better job.

 

[SteveSimpkin]

The Rafael Micro R836 digital TV tuner chip used in the SDS 100/200 is speced for a frequency coverage of 42-1002 MHz so I could understand if it did not perform well below 42 MHz. Uniden did add additional front end bandpass frequency filtering for the various bands the scanner covers. Perhaps they were a bit aggressive below 80 MHz where you are trying to use it.

 

[dazza0768]

The Rafael Micro R836 digital TV tuner chip used in the SDS 100/200 is speced for a frequency coverage of 42-1002 MHz so I could understand if it did not perform well below 42 MHz. Uniden did add additional front end bandpass frequency filtering for the various bands the scanner covers. Perhaps they were a bit aggressive below 80 MHz where you are trying to use it.

Sounds like it mate. Yeah she's pretty crackley or deaf unless used on a discone or if any usb cable is plugged into it. I dont even need the wall adapter or to have it plugged into power.

 

[kb5udf]

Is it possible that the USB cable is providing a needed antenna counterpoise?

 

[SteveSimpkin]

Is it possible that the USB cable is providing a needed antenna counterpoise?

I missed the part about the reception being much better using an SMA handheld antenna with the USB cable attached. I think you are right kb5udf. I suspect most antennas that you would use with a handheld scanner (including the stock antenna) perform poorly at those lower frequencies. Having a counterpoise would likely help.

 

[dazza0768]

Yes it seems thats what the USB cable is doing. I'm just unhappy that the 436 and 325 is good as gold but the 100 is not. Its a real shame.

 

[Ubbe]

I have measured the sensitivity of a SDS100 to be pretty much 0,35uV over it's whole frequency range from 25MHz up to 900MHz. What's come into play are interferencies that the receiver are over sensitive to. I actually have the best reception at 69MHz and 85MHz with a SDS100 of my scanners connected to the same antenna splitter. Be sure to use a FM broadcast filter and set the filter settings to Off for best sensitivity. Also try both NFM and FM modes to see what works best.

/Ubbe

 

[dazza0768]

I have measured the sensitivity of a SDS100 to be pretty much 0,35uV over it's whole frequency range from 25MHz up to 900MHz. What's come into play are interferencies that the receiver are over sensitive to. I actually have the best reception at 69MHz and 85MHz with a SDS100 of my scanners connected to the same antenna splitter. Be sure to use a FM broadcast filter and set the filter settings to Off for best sensitivity. Also try both NFM and FM modes to see what works best.

/Ubbe

Yes i had the issue where FM broadcast radio stations were screwing with the BCD436HP so I purchased an FM band stop thing. Works great now.
With the 436 using a diamond RH77ca antenna it works great, no fm blocker needed but same antenna with the usds100...no go, doesn't have very good reception at all. 2 bars if I hold it in the right position. Otherwise once I plug a USB cable in, it earths the signal out a bit more so I can receive clearly.
It works fine on the discone without the fm band blocker....just struggles with mobile antennas.

 

[jonwienke]

It is good as gold when plugged into my discone via coax but any hand held antenna doesn't work good at all. Its also good as gold when the power cable is plugged in, even if its just the USB cable without power.

You've just proved the opposite. If the receiver was the problem, it wouldn't ever work well, even with the discone connected. You're using your RH77CA out of band and somehow expecting good reception. The same goes for the SDS200; the telescoping factory antenna drops off below about 110MHz when fully extended, as does the RH77CA. The receiver isn't the problem, it's the antenna.

The SDS100 has a smaller RF circuit board than the older models, so you've got less counterpoise for a whip antenna that requires a ground plane. It's not an issue at higher frequencies, but will attenuate lower frequencies more, unless you're using a dipole antenna that doesn't require a ground plane, or an external antenna connected with coax.

 

[dazza0768]

Roger. Yeah points me back to the scanner. As I'm comparing it to the 436 which works great on the same whip. No worries. Thanks for your guys comments.

 

[jonwienke]

Roger. Yeah points me back to the scanner.

Nope. It points to using a monopole antenna out of band with insufficient ground plane. The SDS100 would get better low band reception in your situation if you made it 2-3x as big (to give it mor counterpoise), but I suspect that would trigger an entirely different set of complaints. The correct solution is to find or fabricate a dipole antenna tuned to the band you're trying to monitor. Do that, and it will work great regardless of which radio you attach it to.

 

[Ubbe]

The SDS100 would get better low band reception in your situation if you made it 2-3x as big (to give it more counterpoise)...

What about that huge SDS100 battery? isn't it enough metal in that to make it a bigger counterpoise than the 436? Or is it just chemicals and plastic?

/Ubbe

 

[jonwienke]

What about that huge SDS100 battery? isn't it enough metal in that to make it a bigger counterpoise than the 436? Or is it just chemicals and plastic?

Even with the battery, the SDS100 has less circuit board area than the 436. The RF board in the SDS100 is far smaller than the 436, about the same dimensions as the display.

 

[dazza0768]

My end point would be that you'd expect a very expensive top of the line scanner to be much better than its previous sisters. The 436 can listen to our 75mhz fire frequencies on an 800mhz remtronix antenna clearer than the SDS100 picks up most stuff on the rh77ca.
I have the discone so that keeps everything in line when I'm at home, but when I go mobile I throw the expensive sds100 away for the 436 or 325.
The sde100 just has too many flaws for what it is.

 

[jonwienke]

If it was the scanner receiver, as opposed to the antenna, it wouldn't work even with the discone antenna. The problem is that the SDS100 is smaller than the 436, and has less counterpoise available when using an antenna below its designed frequency range. Its the user, not the radio.

 

[dazza0768]

If it was the scanner receiver, as opposed to the antenna, it wouldn't work even with the discone antenna. The problem is that the SDS100 is smaller than the 436, and has less counterpoise available when using an antenna below its designed frequency range. Its the user, not the radio.

Well I'm not using a 1m long antenna just because the scanner isn't good at that freq range without extra help that most other scanners don't need.
Step back and have a look at the situation....uniden could have done it alot better than they did.

 

[jonwienke]

You've already proved that isn't necessary if you plug on a USB cable to be a counterpoise. It's either that, or revert to a physically larger case and circuit board layout. Which you'd complain about, if they did that.

 

[dazza0768]

Even with a 1m long extendable telescopic antenna it doesn't work.
It is necessary for a mobile antenna for a counterpoint in the form of the USB cable.
I wouldnt complain if it was larger, to be honest as it will work a bit better when mobile, I enjoy the size of bugger scanners. Thank God for discones huh
Its a shame our fire band is down in that area of the spectrum.

 

[jonwienke]

Even with a 1m long extendable telescopic antenna it doesn't work.

Because it's a monopole antenna that requires a ground plane the scanner isn't physically large enough to provide. A loaded dipole (which doesn't require a ground plane) wouldn't have to be a meter long and could still perform better.

 

[ratboy]

Do they still sell that "Tiger-Tail" ( I might have the name all wrong) wire that went over your BNC to make a counterpoise? I copied it with a copper washer from my junk drawer, and a 18" hunk of wire, and it helped a lot.