Antenna testing and more confusion

[paulmohr]

Ok, I have more antenna questions, go figure right. I will try to explain everything in the best detail I can.

Basically what I want to know is if there is a way, and if there is, how would go about testing an antenna system without fancy antenna testing equipment ( or basic for that matter lol). Like is there a way to test the receive capability using a basic multi meter ( I have a fluke 77 and a southwire 21050T with a clamp) or the built in bandscope feature on the 325p2. I don't have a SWR meter or anything like that. I can find information for testing antennas, but it pretty much all pertains to testing transmit, not receive.

I am asking because I have tried 4 different antennas, 3 from various locations, heights, and feed lines. These would be a Discone 130NJ, a home made off center dipole and a home made yagi tuned to 850 mhz. The 4th being my large TV antenna on a 30 foot tower. And obviously the stock stubby that came with it. All of the outdoor antennas give similar results, not exact, but similar. The best is the Discone 30 feet in the air with new RG6Q cable and compression fittings. I can pick up analog signals from between 30 and 50 miles away with any of the outside antennas at a reasonable height.

What I am wondering is how would I know if any of these antennas were performing as well as they should. Or are they performing no better than a random chunk of metal 15-30 feet in the air. Or is just the length and height of the coax acting as an antenna? I honestly don't know how I would know the difference one way or another. They could all work above average, average or crappy, all I know is they are similar based on what I can pick up. Like I said, I like the discone the best, but it isn't a "Holy crap, that antenna rocks!" kind of a situation if you know what I mean. And for all I know that system works better simply because I have brand new RG6Q coax running 30 feet in the air with a pre amp at the top. For all I know I may have a 100 dollar antenna just sitting up there looking pretty lol.

The reason I am asking and doubting the set up is because I have a couple guys in the local MI forum telling me I should be picking up a system I can't. They live farther away from the coverage zone then I do, and they can pick it up fairly easy. They live on the other side of the system than I do though. I live to south of it, and they live to the north/ north east of it in the detriot area. I don't know if that makes a difference or not to be honest.

If I get in the car and drive for a little ways to get in the coverage area I can pick them up with the stock antenna on the 325p2 inside a car. They seem to believe that if I live that close to the coverage area I should be able to pick it up pretty easy with an outdoor antenna. Which would lead us to believe I had an antenna issue, cable issue or connection issue. I have used all these antennas on different feed line from 6 feet, 25 feet and 30 feet. Except the large tv antenna, I can't get to that to swap out cable. And I have tried running the cable straight from the antenna into the scanner, so there were not multiple connections to be worried about. It didn't really make a noticeable difference. I did have one situation where a 6 foot piece of coax in side the house was not right, and it was pretty obvious when bypassed or swapped out. I could still pick up all the local stuff, just the signal strength was about cut in half.

Here is a picture of a map I made with the coverage areas and where I could pick them up in my car when driving around. It also shows were my house is located.

The red circle is the Adrian trunked system, I get that fine obviously.
The Yellow circle is Monroe county Multicast system, which come very close to where I live.
The Blue circle is the Washtenaw Multicast system. This is the one the other guys think I should be able to get based on how far away they live and can pick it up.

The two points on the map that show were I can get a signal in my car are where I can start getting a reliable signal. I can start picking up farther out, but it is spotty, or I only get one ID. I also noticed that the scanner will lock onto an ID or a control channel, but you won't actually hear anything. So just being able to get the signal obviously isn't good enough, you have to be able to get it well enough to actually hear the transmissions. As you can see, I actually had to drive a fair amount into the Washtenaw system to start picking it up reliably.

So you can get an idea of scale, the red circle is 15 mile radius, the yellow is 20, and the blue is 25. At least according to the database. And the distance between Adrian and Techmseh is 10 miles.

Does this make any sense to anyone, or am I just over thinking all of this?

 

[jonwienke]

To compare antenna performance for cheap, your best bet is to get a SDR dongle like this:
https://www.amazon.com/dp/B0129EBDS2/

and download SDR# and observe what you get when you connect various antennas to the dongle. You can see about 2MHz of spectrum at a time, from 25MHz to about 1.7GHz. Make sure you have AGC turned off in the settings so you can make a apples-to-apples comparison.

Don't read too much into other peoples' antenna performance claims. They are prone to the same inaccuracies and fishermens' depictions of the size of fish they caught. Also, if someone lives on top of a large hill surrounded by open fields and has a tall antenna mast, they are going to get better reception than you ever will if you live in a wooded valley. Topography variations make it difficult to make definiteve predictions of what "reasonable reception" should be. The topography between the system and you can be very different from the topography between the system and some other guy on the other side of the coverag3e area. Then there is local RF interference, which is a whole 'nother can o' worms.

Offhand, I'd say you're getting reasonably good performance from your discone, and don't get too wrapped up in weiner-stretching contests with other folks.

One other thing, a rubber duck in a vehicle is the worst possible antenna setup. Even a cheap, average-performing glass-mount antenna like https://www.amazon.com/gp/product/B001DN1L6K/ will run circles around a duck in a truck.

 

[paulmohr]

Guess I will have to get an SDR at some point. I looked into it, but the software part of it scared me off. But then that just might be if you want to scan trunked systems or something. And then I got confused by the price differences, they range from around 10 bucks into the hundreds. What does a 200 dollar dongle do that a 20 dollar won't I wonder?

I guess one way to check if the antenna is working is to just climb back up on the roof and disconnect the actual antenna and leave the cable up there. Then come back down and monitor it to see what the difference is. But man, with the way they designed that diamond discone disconnecting it is a PIA. I will pretty much have to dismantle the roof mounted 10 foot mast, the lower the antenna to the ground and take it apart to get to the cable connection. I just know if I do it up on the roof I am going to loose a bunch of those little screws. That was some pretty poor engineering on their part.

 

[jonwienke]

I wouldn't bother with that. If you're getting anything from 50 miles away, it's working pretty well.

 

[krokus]

If I get in the car and drive for a little ways to get in the coverage area I can pick them up with the stock antenna on the 325p2 inside a car.

This bit tells me there is something near your causing your problem. It sounds like you have interference, or desensitization going on. (The 2nd is when another signal is strong enough to make your radio turn down the amplifier gain, thereby losing the signal you want.)

Are there any transmitters within a couple miles of your house?

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[paulmohr]

There is one of those large antenna towers practically right next to my house. I don't know if that is a transmitter or not. There is also an FM radio station in town about 5 miles away, 103.9 FM. Other than that I am not sure, is there a way for me to look something like that up?

I know when I originally bought the scanner from Herkimer Radio in Monroe MI I asked the guy about getting an antenna. He said there was really no need, since he programmed the scanner for me he knew exactly where I lived and said I lived in the middle of 4 repeaters so reception shouldn't be an issue. And for the most part he was right. I can pretty much get all of the local town fire and PD and my county's trunking system with the little stock antenna.

 

[jonwienke]

The big towers almost always have multiple transmitters; the tower owner rents space to whoever needs a good transmitter location. The one in my neighborhood does cellular and paging, and perhaps other stuff as well. But it doesn't affect reception with the my 436 that I've been able to tell.

 

[krokus]

There is one of those large antenna towers practically right next to my house. I don't know if that is a transmitter or not. There is also an FM radio station in town about 5 miles away, 103.9 FM. Other than that I am not sure, is there a way for me to look something like that up?

There are a few ways. The FCC website has look up tools, based on a radius around a location, frequencies, and service. (In various combinations.)

You can also search the antenna structure database, which can be easier if you can find the registry number for that site. (Which is often on the gate into the site, along with the ownership information.)

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[paulmohr]

So if i had a frequency, or more than one that was interfering with my reception, or desensing my scanner could I use the bandscope mode to try and find it? I would think that if that was happening it would show up as a very strong constant signal on a specific frequency or band of frequencies.

Like for instance if I think it is a cell phone transmitter problem, most of them transmit in the 900mhz range or there abouts. So I could go directly to 950mhz and hold on it. Then enter bandscope mode and adjust it until it was scanning 800 to 1000mhz, or whatever range I desired. Then look for spikes and use the same process to narrow it down to a specific frequency or band of frequencies. like I said, I would think if the signal was strong enough to desensitize my scanner it would be pretty strong and constant, like it would peg the meter out or something.

And once I find the problem spots I would what, get or build some kind of notch filter to attenuate them?

Or could I jury rig my atheros AR9271 network adapter to somehow scan for frequencies? Kind of like a half *** SDR dongle? I know, I could just get a dongle and probably do exactly what I am trying. However that isn't something I can just run to walmart and pick up. I would have to order one with the adapters need to hook up an antenna. Well and then learn how to use it lol.

 

[prcguy]

Yes on the band scope for finding potential interfering signals. Commercial radio guys use a spectrum analyzer to correlate signals that come and go with interference to pinpoint the source. I've done this to find problems plaguing repeater inputs.

You have to be careful and use a spectrum analyzer or receiver with spectrum scope that is not prone to overload, otherwise your just looking at your own receivers internal problems. Most dongle SDR receivers are worse than a cheap scanner in this regard and will create their own internal IMD from strong signals where there is no actual interference on the band.
prcguy

So if i had a frequency, or more than one that was interfering with my reception, or desensing my scanner could I use the bandscope mode to try and find it? I would think that if that was happening it would show up as a very strong constant signal on a specific frequency or band of frequencies.

Like for instance if I think it is a cell phone transmitter problem, most of them transmit in the 900mhz range or there abouts. So I could go directly to 950mhz and hold on it. Then enter bandscope mode and adjust it until it was scanning 800 to 1000mhz, or whatever range I desired. Then look for spikes and use the same process to narrow it down to a specific frequency or band of frequencies. like I said, I would think if the signal was strong enough to desensitize my scanner it would be pretty strong and constant, like it would peg the meter out or something.

And once I find the problem spots I would what, get or build some kind of notch filter to attenuate them?

Or could I jury rig my atheros AR9271 network adapter to somehow scan for frequencies? Kind of like a half *** SDR dongle? I know, I could just get a dongle and probably do exactly what I am trying. However that isn't something I can just run to walmart and pick up. I would have to order one with the adapters need to hook up an antenna. Well and then learn how to use it lol.

 

[paulmohr]

Yes on the band scope for finding potential interfering signals. Commercial radio guys use a spectrum analyzer to correlate signals that come and go with interference to pinpoint the source. I've done this to find problems plaguing repeater inputs.

You have to be careful and use a spectrum analyzer or receiver with spectrum scope that is not prone to overload, otherwise your just looking at your own receivers internal problems. Most dongle SDR receivers are worse than a cheap scanner in this regard and will create their own internal IMD from strong signals where there is no actual interference on the band.
prcguy

So if something is overloading my scanner using the built in bandscope probably won't work and I need some kind of special expensive piece of test equipment to figure it out?

 

[prcguy]

If the radio with band scope is being overloaded causing internal IMD then the band scope will show ghost signals that are not really there. You could try attenuating the input to see if the interfering signals go away, indicating they were probably generated inside your radio. If they are still there and weaker with attenuation, but still in proportion to other known signals then the interference is probably external and real.

IMD is a non linear function, meaning if whatever is causing it goes up by 1dB, the IMD grows by 2dB. If you see an interfering signal 30dB above the noise floor of your band scope and you attenuate the receiver input by 10dB, the interfering signal will go down by 10dB if its real and external to your receiver. If the same interference goes down into the noise floor under the same conditions it was probably created inside your receiver.
prcguy

So if something is overloading my scanner using the built in bandscope probably won't work and I need some kind of special expensive piece of test equipment to figure it out? And now I just discovered this thing won't search anything above 960MHz. The specs say it will do 1240-1300, but if I try to enter anything in that range it just says "out of band, press any key".

 

[paulmohr]

Alright guys, it looks like I have multiple issues going on here. I for sure have some sort of cable, connection or antenna issues. While I was scanning frequencies in chunks of 20 MHz at a time to see if any particular frequencies were really strong I noticed some odd things happening. My idea was I would run the test with no antenna attached. Thinking that if something was REALLY strong it might grab it with no antenna. Then I was going to do the same test again with the stock antenna, and then the outdoor antenna and various other antennas to see if there was difference between them. My thinking was that if it was an interference issue, or overloading that weaker antennas would pick up different signals than the larger one. Like if it was overloading issue I might see some frequencies be fairly strong with a small antenna. Then with the larger antenna some frequencies might be really weak or vanish because the radio was self attenuating because of the over load. I am not sure if that is how it works, but it made sense in my head anyway lol.

When I switched from the stock antenna to the outside discone I noticed my signal strength was not constant anymore, it would go up and down.

I removed all the amplification from the system and started running just straight lines to different antennas. I ran the line that comes off my discone up on the roof into the cable I run into my scanner. So, connector attached to antenna, 30 feet of coax, f connector at end of line, one male to male F connector, F connector, 3 foot section of coax terminating in a BNC connection.

This set up was pretty much not any better than the stock antenna for the handheld. And the signal strength was changing. I figured maybe the cable or connections going into my scanner might be faulty which would explain a lot. Unfortunately that is not the problem. I have a pole that I can telescope to about 20 feet so I started putting antennas and coax on it and running it up in the air. This confirmed my worst fears. The discone and RG6Q is no better than a random chunk of metal, or home made antenna put 20 feet in the air. Or just RG6 attached to the pre amp and ran up in the air, with no antenna connected at all.

So something is up with the coax, connection to or the discone antenna itself. Looks like I will be climbing up there and pulling it all down again.

My birthdays always suck, I should have just stayed in bed today lol.

 

[jonwienke]

Your test is invalid. Atmospheric noise level is not constant on all frequencies, and will appear even higher in frequency ranges where the antenna is resonant. If noise level is constant, that guarantees that all you're hearing is the noise floor of your receiver. The bandscope in scanners is really limited and not useful for what you are trying to do.

Get yourself a SDR dongle like the RTL-SDR stick on Amazon (it has the same SMA connector as your scanner), download SDR# for free, and learn how to use it (hint: there is a RR forum specifically dedicated to SDR stuff). You can download SDR# for free and install it and get familiar with a lot of the options before your SDR stick arrives. Do this before screwing up an antenna installation that probably doesn't have anything wrong with it.

 

[paulmohr]

So the signal meter (little bar graph at the top of the screen like a cell phone) on the scanner, the signal strength bar and the RSSI numbers in the log in freescan mean nothing? The fact that one antenna system will give me 2 or 3 bars and fluctuate on the scanner. And will show about 50 to 60 percent signal strength and have RSSI numbers in the 500-600 range. Then I can switch to another system, or swap out a cable and the signal meter on the scanner will show full bars at full strength, the signal strength in freescan will almost peg out, and the RSSI numbers will jump into the 900's. None of this information means anything and can't be trusted? Even though the numbers are repeatable?

We can forget about the whole bandscope test and theory. Even if it did work it isn't anything I could actually do until I verify all my cables and connectors are good.

And I am pretty sure my 325p2 has a BNC connector on it. SMA is just like a little F connector isn't it? Like on a wi fi antenna or something.

 

[paulmohr]

Ok, I just answered my own question. Apparently none of it means anything because I just hooked the Discone back up with its own connection for the scanner so I can swap them quickly. Comparing it to the other antenna the discone was actually picking up more signals from farther away, even though all the numbers, bars and graphs were lower and would indicate it didn't work as well. Swap antenna leads real quick in the middle of a call from a distant source and I would lose it. Even though when using that antenna all the numbers and bars would indicate it works much better. Which just makes no sense to me at all.

Apparently I am dumber than a rock when it comes to this stuff and I have no clue what I am doing lol. Maybe I should take a course in this just so it makes some kind of sense. Even if I flunk it I might learn something. I have tried to research on the internet but I can't seem to find anything that explains it well, or at all.

 

[jonwienke]

If you can pick up traffic from a transmitter 50 miles away, your antenna system is good. Not necessarily perfect, but you can rule out gross errors like a shorted cable or broken wires with a reasonable degree of confidence.

SMA is similar to an F connector, but smaller. The 436 has an SMA connector, but comes with a BNC adapter which most people use unless running the stock antenna. I thought the 325P2 was the same, but apparently that is not the case. You can put a BNC-SMA adapter on the SDR if you get one to have a consistent BNC antenna connector on everything.

The bandscope function in the scanner is useless because it is not detailed enough. Knowing you have a strong signal in a frequency range doesn't help if you can't distinguish between an actual signal and a noise source. A 5- or 6-bar graph for a frequency range won't show you the relationship between a given signal's strength and the adjacent noise floor.

SDR# has a detailed display that displays about 2MHz of bandwidth at a time, and makes it easy to distinguish between high background noise levels and actual signals. If you have a transmitter tower nearby, SDR# + a dongle will help you track down any strong nearby signals that may be interfering with reception or overloading your receiver. The RTL SDR sticks have a manual gain control so you can adjust RF gain manually. When searching, you'll want to set the manual gain such that the strongest signal in your search range (say 25-1000MHz) peaks at about -10dB on the display in SDR#. That will help cut down on overload. You're most likely to find strong signals in FM broadcast, pager, TV, and cellular bands. I'd start by tuning to the strongest local FM broadcast station you can find, and then set the RF gain so that the signal peaks at about -10dB. Start with RF gain at minimum, then increase it until the signal peaks at -10dB.

Once you've surveyed your local RF environment, you'll be better informed about things like whether you need amplification, or if your amplifier is getting overloaded by a strong signal and spamming the output with intermodulation and splatter or noise. You'll be able to see the difference between the signal level for frequencies you monitor with the scanner compared to the local noise floor, which is something the scanner band scope won't show you.

The best way to judge antenna performance is not by how high the peak signal strength is, but by the height of the signal peak compared to the noise floor on each side of the peak. An amplifier will raise the signal peak, but if it raises the noise floor on each side of the peak just as much as the signal peak, you haven't gained anything. Boosting the signal by 20dB isn't helpful if doing so boosts the noise floor by 25dB. And if you're increasing the numbers by picking up RF hash from your computer, that is even worse.

The screenshot below shows SDR# tuned to a local FM broacast station. The top section is the spectrum analyzer, which you can see is far more detailed than what the scanner can display, and is showing a bit over 2.2MHz of spectrum. I parked the mouse on the section of the band being tuned to make SDR# show the signal and noise floor stats. The middle section is a waterfall display, which is basically the signal strength in the top display over time. The bottom section is the channel bandwidth--basically the highlighted section of the top display magnified horizontally.

The station I've tuned to at 92.1MHz has a signal strength 19.5dB above the noise floor, and it comes in pretty clear. You can see another station at 91.7MHz which is about 9.5dB above the noise floor. It comes in OK, but has noticeably more static. If you look at the waterfall, you can see that ther are also stations at 91.9, 92.3, and 92.5MHz, but they are only 3-5dB above the nouse floor, and are too staticy to hear intelligibly.

The peak signal strength of the stations doesn't matter, what matters is how high the signal peak is above the noise floor. The greater the difference, the better your reception will be. Any adjustments or changes to your antenna configuration must follow this principle, especially when it comes to amplifiers. Amplification does no good if it increases the noise floor just as much as (or more than) the signal peaks.

 

[paulmohr]

Ok, that makes a little more sense now. Thank you for explaining it to me, and using pictures lol. The whole noise floor thing was throwing me off and confusing me. I think I get it now.

It is sort of like altitude in an airplane. Knowing your altitude really doesn't do you much good if you don't know the altitude of the ground or terrain around you. Your gauge might say you are 1500 feet up, but if the ground below you is 700 feet above sea level your altitude relative to the ground is much lower.

The radio signal is the same thing I guess. I was looking at it like everything was level and it isn't. The noise floor is kind of like the ground. Knowing how "high" my signal is doesn't do me any good if I don't know where the floor (ground) is. And none of the tools I currently have are showing me this. So I am basically trying to fly without any maps, tools or being able to actually see where the ground is relative to where I am at. I am not a pilot by the way, it was just the best reference I could come up with. If I was a pilot I would probably have a better idea of how radios worked lol.

So I am only seeing part of the picture, and it isn't the part I need to see. I will have to order me an SDR I guess and play with it. From the sounds of it my pre amp is probably just giving me "pretty" numbers in Freescan, but most likely isn't actually extending my range any.

 

[RF-Guy-El-Loco]

You have run into the same problem as I did. I'm baffled. Here's what I "believe" is going on. It isn't a real solution, just ideas to chew on.

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.

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. The stock duck antenna has no coax loss, it is just antenna fed to scanner. The coax is playing games with signal cancellations. You're getting 10 or more trunk channels in the system that are very close to each other in frequency and these signals are turning on/off at random intervals. It isn't just one frequency we want, it is many close to each other of fluctuating RF strengths. I noticed if I so much as moved my RG6 coax around, signal strength at 850Mhz would fluctuate. The coax itself had become my antenna. I tightened the connectors as much as I could, this helped slightly. I then disconnected my antenna and still had the same signal strength and 850Mhz reception quality just on 25 feet of pure RG6 coax with No Antenna at all. Sound familiar?

3. You may have strong cellular signal interference coming off those towers near you. It could be generating a fair amount of (intermod distortion) IMD or it could even be a down converter of something on that nearby tower that's leaking stray RF out. Or an 950Mhz STL link, the radio stations love to use these to shoot their signal to the tower. And it could be the cellular antenna/signal combiners on that tower spraying stray RF out to confuse the scanner. But even if we rule out all the interference, you need to get the most signal and the least noise to the scanner. Part of the RF signal fluctuations are just plain atmospheric variables which really cause P-25 reception havoc.

4. My best 850Mhz cut antenna (a 7ft collinear) still has a fair amount of fluctuations and many signal fades even with a good amount of signal gain. My Yagi works great, but has the same fluctuations fade issues as the others. The RS 800Mhz antenna will mostly not fluctuate, so I put one in the attic on 25 feet of RG6 and it works really good that way. I also have the 2nd best on 850Mhz on an RS multi-band scanner antenna in the attic (#20-176 not being made anymore). The signal fluctuations affect all the antennas mostly the same at the same times, doesn't seem to even matter which antenna is used. FYI. Any signal loss at 850Mhz is too much signal loss. This is all I know.

Motorola should start paying us to do their field strength testing! I've heard the techs on the radio...they go around in and out of buildings all over testing RF signal strengths and mumbling why it doesn't work like it should according to plans. So don't feel dumb, the radio techs are doing trial and error field testing just like we are and scratching their heads a lot of the time too.

 

[jonwienke]

It is sort of like altitude in an airplane. Knowing your altitude really doesn't do you much good if you don't know the altitude of the ground or terrain around you. Your gauge might say you are 1500 feet up, but if the ground below you is 700 feet above sea level your altitude relative to the ground is much lower.

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.