Improving VHF reception for RTL-SDR

[bostonbean]

I work near a hospital in an office building. My office is on the 15th floor, and mostly glass (though it may be UV blocking somewhat). I'm trying to listen to a frequency used by ambulances to talk to the emergency department. It's on VHF, at 155.340 Mhz. I can easily hear the transmission from the ED (which I'm assuming has much higher power), but I can only rarely hear the ambulance end. I'm using an RTL-SDR v4 dongle using the included dipole antenna with a total antenna length of 3 ft (based on a dipole calculator). Is there anything I can do to improve my situation? I.e. a better antenna or switching to an AirSpy HF+ Discovery or adding an LNA? I can't make a hole in the window or put the antenna on top of the building (though I'd love to). Would love any suggestions! Thanks so much.

 

[mmckenna]

A lot of that UV blocking glass uses metallized film in it that effectively blocks RF. The fact you can hear the hospital is likely because you are so close and it's a strong signal.
The ambulance will be farther away, using an antenna with less gain, and might be on the opposite side of the building from where you are.

Short of getting the antenna outside, there's likely not much you can do. Sounds like you already have your antenna adjusted to approximately the right length for VHF. If it's close to the window, not much else you can do.

 

[BinaryMode]

I imagine they use a repeater, right? Maybe on the roof? Because it sounds like since you can hear the ED perfectly fine and the ambulance not so fine you're listening to the input frequency and not the output frequency of the repeater.

What hospital and location?

 

[wgbecks]

I imagine they use a repeater, right?

155.340 is a well-established simplex frequency used for hospital to ambulance communications. I would suggest that the OP is also
subject to a very high noise floor in addition to the answers provided by @mmckenna.

 

[spongella]

Great question with excellent comments. First off is your dipole vertically oriented?

I've used the RTL-SDR earlier versions, mine is the older .v3, a good performer on VHF/UHF. As for the Airspy, I have the HF+ Discovery, works great too but it'll cost a lot more than a dongle. It does have an adjustable gain though. Since it's more geared toward the LF/HF/SW listener, it might be overkill for you.

There's one other you might want to check, which I also own and isn't much more expensive than your current SDR dongle - is the Flightaware ProStick which has an adjustable gain. While it's geared towards ADS-B monitoring, I found it to be excellent for VHF/UHF. It actually outperforms the .v3 version on UHF, at least in my situation. Not sure if they're still available though.

Lastly, buying/making a VHF antenna with more gain might help. Good luck, hope this helps. Merry Christmas.

 

[kb5udf]

I suspect your current rx antenna is not much different from a vhf HT. I have had a few situations where the HT was not very usable, but an external antenna like a 1/2 wave mag mount on a file cabinet gave usable results. Obviously there are other options like various base antenna models that might work as well. The fact that you are high up and get some reception gives me a little hope you can do better with a good antenna, assuming your noise floor is ok, but certainly no guarantee.

A better receiver may be helpful as well (e.g. one with a much lower noise floor) and as another reader pointed out an airspy discovery is an excellent choice, or consider a cheap, surplus commercial radio.

Lastly, depending on how much clout you have or friends you have in the hospital have you considered feeding the audio from the receiving radio (which is probably at a nurses station in the ER). That audio could potentially be sent out over wifi with a little work and again approval from the powers that be in your hospital.

 

[Ubbe]

Those window glasses could be UV and then also RF blocking, but concrete or brick walls might pass RF signals more easily. Sometimes the wall isolation have some moisture protection film that are metallic and will also block RF.

Try and find a VHF signal that are a more constant transmitting one, maybe a digital control channel or even the NOAA channels at 160MHz. Try and find a place where you get the strongest signal and that would be the place for the antenna when you listen to those ambulances.

If a window can be opened then use one of those flat piece of coax cable extensions to go thru a closed window. Anything outside a window will work better than indoors, even a piece or electrical wire connected to the center lead hanging outside the window.

/Ubbe

 

[slicerwizard]

Is there anything I can do to improve my situation?

If you had access to a VNA, you could ensure that your antenna is resonant at the target frequency.

How bad is the noise floor in the spectrum display? If the windows are seriously attenuating RF, you might be able to employ more gain with a good preamp, but I wouldn't expect stellar results.

The best solution would be to find a good deal on a used professional (Motorola, Kenwood, ...) VHF radio that can be tuned to the target frequency, combined with a tuned dipole. The good gear has hot receivers with high dynamic range to fight overloading.

 

[WB5UOM]

And it also depends on what other buildings are around the one you are in and are they as tall, or taller than you on the 15th floor.
155.340 simplex will be greatly attenuated by those other buildings (depending on which direction said ambulance is coming from) as well as the building you are in
Then add the mentioned noise created by all sorts of things where you are and whats near you...
I would not expect much.

 

[bostonbean]

Thanks, everyone -- this is really helpful! I talked to some people in the comms office and it's not a repeater -- it does just transmit and receive on 155.340 simplex. Their antenna is much better than mine, and outside. I can hear NOAA radio really clearly in my office, and I can also use SDRTrunk to hear P25 Phase II digital audio, which is very clear (for the few signals that aren't encrypted -- including the hospital police radios), but I'm guessing that's because they're digital. In terms of other buildings, there's pretty much a direct line of sight to the hospital, but a lot of other office buildings in the area, so that's likely not helping. My dipole antenna is vertical -- I'll try to move it around my office a bit, and maybe explore different antenna and dongle options. Grateful for everyone's help and ideas!

 

[LimaZulu]

Changing equipment won't help much unless in terms of selectivity (which is questionable from one to another device) which will be only helpfull if a lof of other signals are in the way.
The best solution is always a propper anenna. If you only want to monitor this frequency, just calculate the propper lenght for the exact frequency. Or maybe some diy directional antenna will be better approach.
Addind LNA isn't going to help as well because it will not only amplify the desired signal, but all others.
All in all my first approach will be antenna adjustment (length and position) and maybe some filters if a lot of background noise is present. The filer could even be calculated to cut all otsude desired frequency, of course if you only want to monitor this exact frequency.

 

[dlwtrunked]

I work near a hospital in an office building. My office is on the 15th floor, and mostly glass (though it may be UV blocking somewhat). I'm trying to listen to a frequency used by ambulances to talk to the emergency department. It's on VHF, at 155.340 Mhz. I can easily hear the transmission from the ED (which I'm assuming has much higher power), but I can only rarely hear the ambulance end. I'm using an RTL-SDR v4 dongle using the included dipole antenna with a total antenna length of 3 ft (based on a dipole calculator). Is there anything I can do to improve my situation? I.e. a better antenna or switching to an AirSpy HF+ Discovery or adding an LNA? I can't make a hole in the window or put the antenna on top of the building (though I'd love to). Would love any suggestions! Thanks so much.

Since I did not see anyone else mentions...What very strong signals are near you? Are any antennas on the roof> Is an F< broadcast filter near you? (Use a good FM broadcast band notch filter). Your receiver may be de-sensed by them--note, if this is the case, you will not hear the the station de-sensing you when you listen to the frequency that is being de-sensed so this is often not realized as the cause..