Receiver specifications

[KMA440]

I have been using a Uniden 780 scanner for the longest time to listen in on my local airports. It seems to do better than my 785D or even my 796. It suffers only from low audio output from am.

But I'm really curious why the sensitivity is only one microvolt. I know enough about radio to get my technician license not long ago and I can kind of understand when they're at 30,000 feet talking to ATC 1 microvolt sensitivity should be fine and should match their low output power I think it's 12.5 pep or 25 watts AM. I don't think you can do Side Band on them it's just their way of writing it.

But what's the deal with limiting the receiver sensitivity when we are taught antenna, antenna, antenna. A good antenna system almost always overcomes a poor transmitter system.

If any of you are involved in the actual maintenance of avionics at an airport, could you tell me what VHF antenna they tend to lean towards that puts its signal up into the sky rather than at the horizon? I'll need to save my money for when I'm sure but I'd love to be able to hear as well as they do.

Thanks.

 

[ko6jw_2]

First of all you might consider a discone antenna. They will have a flat response across the air band (mil air too). They have a relatively high angle of radiation. Higher gain antennas will have narrower response and a lower angle of radiation. Not so good for air band.

Sensitivity in AM receivers is not "limited" it's just measured differently than FM. You need to look at the specified s/n ratio as well as the micro volts. A Yaesu air band HT advertises a sensitivity of .8 microvolts for a 6DB s/n ratio. Another radio specifies 1 microvolt for a 10DB s/n ratio. What's the difference? You won't notice. All things being equal the difference between .8 and 1 is about 2DB (volts), but the difference in s/n ratio might mean that the 1 microvolt radio is a little more sensitive.

No sideband on VHF AM. HF aircraft bands use upper sideband.

 

[majoco]

Download this...

https://tf.nist.gov/general/pdf/2304.pdf

... and skip everything and go the last paragraph of "Conclusions" and you will see that the limiting factor in VHF communication is cosmic noise - not receiver sensitivity or antenna gain - so any sensitivity better than 1uV for 6dB for S/N+N is just a waste.

 

[Ubbe]

Towers and the remote sites usually have standard GP antennas or dipoles. Some towers have discone antennas as a backup spare or for non critical monitoring. If aircrafts are high in the sky they have line of sight to the antenna and even a paper clip would work. You can hear the ISS satellite 40km above you using any kind of antenna. But when an aircraft are on low altitude it could be obstructed by terrain or buildings and they need all the signal they can get. So antennas are standard ones that are in nature peaked at the horizon.

You can modify the 780 by soldering two component legs together to increase the gain in the AGC circuit and give a more balanced audio level in AM mode. I haven't noticed any negative effects. The 780 have a tracking front-end filter in the HI-VHF band that follows the frequency you are monitoring, like professional receivers, that will improve reception. I don't know of any other Uniden scanner that have that feature.

/Ubbe

 

[KMA440]

Download this...

https://tf.nist.gov/general/pdf/2304.pdf

... and skip everything and go the last paragraph of "Conclusions" and you will see that the limiting factor in VHF communication is cosmic noise - not receiver sensitivity or antenna gain - so any sensitivity better than 1uV for 6dB for S/N+N is just a waste.

I really wish just asking a basic question about sensitivity. That article was written in 1948. We no longer use bipolar transistors for amplifiers. We also use multiple stacked band pass filters to narrow the frequency range down. Now we're beginning to see the pendulum swing all the way to the other side with SDR.

If you follow the logic of the information you presented to me you're going to need explain to me so then then how come marine radios and Motorola radios did in police cars all have sensitivity down at .25uV or better.

I am balancing my flying experience, which isn't much, only 136 hours, and knowledge of radio together. The radios are capable of greater sensitivity and I suspect that my initial thought was correct. The receiver strength has been decreased so that it matches the transmitter output of 12.5 watts or 25 watts pep. Neither an alligator nor an elephant be.

We use NXDN for plant operations and security where I am at. I work for a government contractor. I noticed that even outside of a building in the clear our NXDN will be nothing but garble and the dispatcher will ask us to repeat several times. That's just digital. I suppose the antenna system could be corrugated hard-line inside the building in order to put signal down into the sum of these Subterranean levels.

It was because of that that I started looking at the specifications of our Motorola handy talkies and the radios in our mobiles. I looked and found their specifications to be .20uV. We are on VHF and I would assume that the cosmic noise identified in 1948, 71 years ago, would not affect us. Roswell happened in 1947. There are DSP firmware algorithms and noise filters in our radios that were not available in 1948. Cosmic Noise? So it's not a what's it called? Apples to Apples comparison.

 

[ko6jw_2]

There is a fundamental difference between AM and FM. You want to know why an AM aircraft receiver has lower sensitivity than an FM two-way radio. First of all, is this really true? We are talking about two different modulation systems and two different receiver designs. I don't care what era you're talking about. FM sensitivity is specified as a certain number on microvolts for a certain level of quieting in decibels. Thus, we might say the sensitivity is .5 microvolts for 20db quieting. This is simple to measure. Take an AC voltmeter and adjust the volume of the radio to read 0db. Now apply a signal. The amount by which the meter drops is the quieting. There are some different ways of measuring quieting, but we don't need to complicate the discussion. FM inherently reduces noise. Noise is AM

In an AM receiver sensitivity is measured was a signal to noise ratio with a modulated signal. Thus, we could say that the sensitivity of an AM receiver is .8 microvolts for a 6 db signal to noise ratio with a 1000Hz tone at 30% modulation. The noise floor will determine the sensitivity. Band width is also a factor. Single sideband and CW have a lower bandwidth that therefore better signal to noise ratios.

If you wanted to increase the sensitivity of an AM receiver by adding more RF amplification you would also amplify the noise. Would this change the signal to noise ration. No. DSP does not increase sensitivity - it just reduces noise. However, this noise reduction also affects the signal to some extent. Too much DSP and the signal sounds like mud.

This is why most two-way radios are FM other than aircraft. By the way, all digital modes including NXDN are FM.

Do you really believe that aircraft radio manufactures deliberately make radios with less than optimum sensitivity? Why would they do this?

 

[majoco]

I know it's an older aircraft transceiver but it still is in common use on larger transport aircraft. Collins 618-M1 - Receiver sensitivity specification - S+N/N NLT 6dB for 3uV 1000Hz 30% modulation, 100mW audio output for 4uV 1000Hz 30% mod. Transmitter output power NLT 25 Watts. These figures are those that have to be met after bench repair or scheduled maintenance. Aircraft antennas are not the best, often they are a shortened quarter wave ground plane enclosed in fibreglass on the bottom and top of the aircraft and usually arranged to have their best response from the forward direction. ILS and VOR signal strengths at the intercept altitudes have to be over 5uV at the receiver antenna terminals so the actual signal strength to a half wave dipole is likely to be considerably higher than this. (ICAO Doc 8071)

One of the advantages of FM is that it is not dependant on the amplitude of the signal at all, so you can actually have a signal that is below the noise level and still make sense out of it - amplify the signal, clip off the tops and bottom of the waveform in a limiter, the frequency deviation remains which you can convert into audio in the discriminator or phase sensitive detector. You will get a sensitivity better than 1uV. Interestingly, the received signal from a GPS satellite overhead is about -130dBm which is less than 0.1uV in 50ohms.