Antenna testing and more confusion

[jonwienke]

1. Any tiny signal loss of even 1-2 dB in the coax feed line at 850Mhz is a huge loss of 850Mhz signal to our scanners. Noise creeps in. You would have to run Pro grade Heliax or rectangular waveguide or such, even LMR 400 would still be 1-2 dB of loss at 900Mhz.

No. 1-2dB is not a "huge signal loss". You've got some problem other than signal loss going on. I've got about 100 feet of RG-6 quad-shield (8-10dB loss, depending on which calculator you use) between my ST-2 and my scanner, and have little trouble picking up the 800MHz system in the next county.

2. The coax line itself is becoming your antenna at 850Mhz and thus any length of coax at all is going to screw up and interfere with the "Real Antenna" whatever that antenna is.

This is only true if you have crappy poorly-shielded coax, or are dealing with really strong signals. RG-6 quad shield is better-than-average. If your feedline is acting as an antenna, you need better feedline, or you have an antenna issue. Some antennas (e.g. J-pole, LPDA) need a choke balun to prevent the feedline from acting as part of the antenna. A discone shouldn't require one.

 

[prcguy]

Its all about signal to noise ratio. If you have a weak signal that's say 6dB above the noise and difficult to hear, adding a preamp at the radio end can raise that signal by 15 or 20dB, but the noise will go up and the preamp can introduce IMD that further reduces the signal to noise ratio. The end result can be a high signal level but maybe 1dB signal to noise ratio which is no reception at all. Without the preamp you might have 1 bar on your meter and can hear the conversation and with the preamp the signal is now 3 bars but its totally unintelligible.

In ideal setups with a robust preamp and filters at the antenna you can actually improve reception by overcoming coax loss, but that's not a beginner project. For every receiver setup there is a thing called System Noise Figure, which includes every component from the antenna output to the receiver speaker. Every receiver has an internal noise figure which is a combination of loss in front end filters, gain of the front end amplifier, loss of internal mixers, etc. This number will usually fall somewhere between 5 and 10dB.

System Noise Figure is mostly driven by the loss before the first amplifier in the system and the gain/noise figure of the first amplifier in the system. When you place a good preamp right at the antenna with a very low loss filter, or maybe no filter at all, the System Noise Figure is set at a very low level and reception of weak signals is improved. By placing a good preamp and filter at the antenna you can improve the System Noise Figure to maybe 2dB, which is better than the 5 to 10dB rating of the receiver alone. That is assuming the preamp is working well within its ratings and not creating IMD, which will raise the noise floor and create ghost signals that are not real.
prcguy

Bingo. The signal level is the aircraft altitude above sea level, and the noise floor is the terrain height above sea level. The difference between those numbers is the key thing. An amplifier only helps if it increases the aircraft altitude more than terrain height. You can be at 17,000 feet and still crash if the amp throws Everest in your flight path. And you'll have additional problems if you try to fly the plane higher than it is designed to go.

You won't know if it helps or hurts until you test.

 

[paulmohr]

Ordered the RTL dongle and a SMA adapter kit. Amazon didn't have them in stock so use the Worldwide option on RTL's website. I think it is coming from China, so who knows when it will get here.

I still might climb up there and re do the feed line and connections on the Discone. I am not entirely confident in one of the fittings, primarily the one connected to the antenna. It was first attempt with the compression tool and the connectors that came with it on RG6q cable. They were really tight and I ended up not being able to fold the shielding back and still put the connector on. I didn't figure it mattered since the sleeve slides down into the shielding. However I have seen a few videos now where they are saying that is a pretty important step lol. I now have some all metal outdoor RG6Q connectors that are looser on the quad shield cable then the ones I originally used. So I might just dismantle it and start from scratch with the better connectors.

The hard part will be keeping myself from doing it before I get the dongle in the mail. I should really wait until I get it so I can see if I even need the pre amp up there, or if it actually makes my signal worse. It would be a few less connectors I have to use if I don't use it. Not to mention it would simplify things and be less stuff to go wrong in the future.

 

[paulmohr]

I just watched a video with an SDR and the guy cranked up the gain to show you what "ghosting" looks like, pretty interesting. I am going to assume though that anything I see on the SDR won't actually apply to my scanner. I would assume a lot of this has to do with the actual electronics in the device. So if I find ghosting or interference issues on the SDR, that doesn't necessarily mean the same thing exists on my scanner since they are different devices. Or on the flip side, my scanner could be doing things that do not show up on the SDR. So most likely what I am going to be doing with this thing is just looking for really strong signals near me that "might" be effecting my scanner right? After that what do you do? Build or buy some kind of band pass and/or notch filter for those frequencies? Honestly a band pass filter in the range of 150-860 MHz would work for me I think. I don't listen to anything below or above that. But what if I needed to get rid or tone down a specific frequency that fell inside that range, like 162.45 or 455.5? Is that what a notch filter does? And can you combine the two?

 

[krokus]

The hard part will be keeping myself from doing it before I get the dongle in the mail. I should really wait until I get it so I can see if I even need the pre amp up there, or if it actually makes my signal worse. It would be a few less connectors I have to use if I don't use it. Not to mention it would simplify things and be less stuff to go wrong in the future.

Does your preamp have a pass through mode? Maybe when the power is off? If so, enabling that mode would show if the amp is adding problems to your efforts.

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[paulmohr]

I am not sure, it is just your basic RCA pre amp from Lowes. It supposedly has an FM trap and an antenna combiner if you want to use a separate VHF and UHF antenna though. I think if you bypass the power injector it might just pass through, but not really sure. Those are some things I want to test when I get the SDR. I don't think the FM trap does anything to be honest based on listening to fm signals while flipping the switch on and off. I think with the SDR I will be able to get a more accurate look at what it is doing.

For now it isn't mounted on the mast with the antenna, so I can just disconnect it if I want. Like I said I have tried various antennas both with and without several different cable amplifiers. It makes my signal strength go up for sure, but I don't know that it actually increases my range or not.

 

[krokus]

I am not sure, it is just your basic RCA pre amp from Lowes. It supposedly has an FM trap and an antenna combiner if you want to use a separate VHF and UHF antenna though. I think if you bypass the power injector it might just pass through, but not really sure. Those are some things I want to test when I get the SDR. I don't think the FM trap does anything to be honest based on listening to fm signals while flipping the switch on and off. I think with the SDR I will be able to get a more accurate look at what it is doing.

For now it isn't mounted on the mast with the antenna, so I can just disconnect it if I want. Like I said I have tried various antennas both with and without several different cable amplifiers. It makes my signal strength go up for sure, but I don't know that it actually increases my range or not.

Any amp that is not at the antenna is boosting a degraded signal, and boosts any noise present, too. That means the amplifier can make things worse, by raising the noise floor, and/or boosting a signal to the point of desense.

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[paulmohr]

That is something I will test as well when I get the SDR. Like I said I have a pole that telescopes so I can mount the pre amp right at the antenna and run it up to about 20 feet to see what happens. I will also test some other amps I have which are basically cable boosters and distribution amps. All TV and Cable types, not anything actually made for ham radio or scanners. Basically it is just going to give me something to do and satisfy some of my curiosity.

I even went out and got a cheap laptop just to run the SDR and the Freescan software for my scanner. I was going to get a windows based tablet, but the laptops price wasn't that much more. And it has a bigger screen, better specs and better connection options. The tablet pretty much just had a mini usb port that doubles as a charging port. The small laptop has 3 full size usb ports, headphone/mic jack, HDMI output and mini SD card port.

This will give me the freedom to take it outside, or go mobile with it if I want. Something I can't do with my desktop. Plus my mom doesn't drive much anymore so I spend a fair amount of time driving her around and waiting in the car while she shops, visits or does whatever. This will give me something to play with in the car.

If you would have told me 5 or 10 years ago I would be able to get a laptop for under 200 dollars I would have looked at you like you were nuts.

 

[paulears]

A while back up the topic signal to noise was mentioned, and overlooked - it should have said SIGNAL TO NOISE it's that important.

You have collected so many bits and pieces of the science but not quite put them into orders that make sense, that's all.

Signal strength on a meter is simply an indication of relative signal strength. Few meters have any real accuracy now they are digital - they might be 4 or 5 steps top to bottom, and the point they change just picked randomly. They also work differently depending where in the receiver they take their drive from. Imagine an older 25KHz radio signal from maybe an old ham radio from 40 years ago. If you listen to this on a modern radio it's often loud and distorted to a degree - it can even be easier to hear, because the signal to noise appears better, but much of this is due to the receiver design. So a weaker 25KHz channel signal might be better subjectively than a 12.5KHz stronger one. Meters tell you one thing, your ears something else. When doing antenna tests the critical thing is simply relative differences. Few people can measure the signal in microvolts - which would really mean something. One useful thing are antenna switches and variable attenuators, especially ones that have dB markings. You can switch between antennas and then adjust the attenuator so they sound and display on the meter the same reading. You then have a real value for how much better or worse one antenna is to the other. Antenna X is 2.5dB lower than the other. Some people even do it the other way - add attenuation so you cannot hear a signal, then slowly reduce the attenuation until it becomes distinct, with a particular type of noise - anything you can really determine properly. turn the knob up until you can just detect it - then make a note of that setting and do it again on the other and see the difference. As dB's are ratios, your results mean something - one is XdB better than the other. You can use this to check the loss in antenna feeder, if it's swappable. Do the test, record the result and then swap and repeat.

I have a list of permanently transmitting antennas in the district in the VHF and UHF bands that can be heard here. On top of that, I have the local airfields, and harbour stations - All plotted on a single receiver's meter. that way I can compare properly, and often get very odd results. With two different radios, even identical models, it doesn't work, because they are not calibrated together. If you get a ladder out, swap a cable or antenna, then compare it - your memory wrecks the results.