Another stupid antenna question

[paulmohr]

I am wondering what the difference is between three different antenna systems is. I was going to try and draw pictures of them but it ended up looking like a 6 year old drew it after having too much sugar lol. So I will try to explain them instead.

First system would be your basic multi band antenna, sort of like the large tv antenna's you see, but made for radio waves. A yagi antenna for UHF (800mhz) and then a bunch of different length elements to cover the lower frequencies. Basically an antenna that would cover 100-400 and 800mhz. All the elements on the same boom with a 300 ohm twin lead connected to a 75 ohm matching transformer.

The next system would be a Dipole antenna, or maybe a long periodic for VHF and then a directional high gain UHF yagi mounted above it on the same mast. Then connect the yagi to the dipole with 300 ohm twin lead. And then connect the dipole to a 75 ohm matching transformer and running coax to the scanner. Is there any reason why the two antennas wouldn't "think" they were on the same boom?

The third would be the same yagi/dipole combination, but instead of connecting them with 300 ohm twin lead each would have its own matching transformer with 75 ohm coax running into a splitter then running one coax lead to the scanner.

I understand from reading other posts that the third option is a bad idea, basically running two separate antennas to one scanner. What I don't know is WHY this is bad. Does it change the load the scanner sees, does it lower the signal too much, or do they cancel each other out or something?

And what would the difference between system one and system two be? To me it seems like they are the same thing, just not on the same boom is all. Is there some reason system two wouldn't work, but system one does?

System two would be ideal for me (if it works) because I could use the dipole as a non directional 150mhz antenna, which I am using now and it works great. Then the high gain yagi could be used as a high gain, directional 800mhz antenna that I could aim at different trunked site towers. My complete lack of knowledge of antennas is telling me this could probably work. But in the back of mind something is telling me it won't work because of some technical reason I don't understand. Because if it worked, I would see others doing it, and I haven't.

Something is telling me system one is a compromise, but would work and systems 2 and 3 are a horrible idea and won't yield me anything but a waste of metal, money and time. And that the only real good approach is to have multiple scanners each with dedicated antennas tuned for a specific range. Which equates to expensive, and having antennas all over my house lol.

Any input would be appreciated, thanks. Honestly I am just bored and kicking ideas around in my head.

 

[mmckenna]

Phasing multiple antennas together can work, if it's done right. There's a lot of math involved. It's not as simple as connecting random length feed line together. Precisely computed lengths of cable, taking into account velocity of propagation (how fast the signal goes through the wire) for specific frequencies is how it's done.

Done incorrectly, signals can actually cancel each other out.

One thing you can look into is a diplexer. They'll allow joining antennas together for multiple frequencies by blocking the other frequencies from the other ports. This can then be fed into one radio. The diplexers do add some loss, but they make life a bit easier.

 

[lmrtek]

TV antenna companies have already been doing this for many decades so they tend to work pretty well when mounted vertically and most these days have direct 75 ohm feed

When combining separate UHF and vhf 75 ohm antennas you use what's called a vhf/uhf combiner and it has very little loss compared to a splitter

 

[majoco]

TV antennas do work, but they are made specifically for the TV bands although there is a lot of overlap with the VHF/UHF radio bands. A cut-down TV antenna like mine in the pic works well but it certainly is not a long-distance antenna. It has a built-in phasing harness (the horizontal metal bars and the other pair going to the longest dipole) and the balun is in the long black tube.

Discones are reasonable broadband antennas, but they have no gain and are not directional.

A log periodic antenna would work, but to have the frequency range you require (100 to 800MHz did you say?) it would have to be a specially made device with a fair bit of experimentation before it comes right!
It would be expensive and probably fragile too.

Phasing between two antennas with a harness is best for only one frequency per antenna - the phasing length is critical and as soon as you get away from the design frequency then signal losses mount up - the other cable looks like a short circuit in the extreme case.

Eventually it's cheaper to have one antenna per one receiver!

 

[RF-Guy-El-Loco]

Somewhere in there, you hit upon your own good logic. You want one of your 800Mhz antennas to be Directional, this calls for a Yagi to point at one tower, so you can get the most signal possible and null out anything else. But it will probably still be able to get the local 800Mhz tower enough to hear it but with severly reduced level. This may work. For the 155Mhz and 460Mhz bands, a good Omnidirectional antenna (J-pole, log, multiband, or discone) is what you want there. You can get away with this combining on one antenna setup, (no harm to try) but it will present a weird frequency reception pattern to the scanner and confuse it and that will then confuse you, what Mmckenna described. A diplexer is the professional solution, but they are costly for the hobbyist.

Remember, with the tv antenna booster, it also amplifies the general RF noise that you don't want along with signal you do want to get. And you get overload. A chunk of those higher RSSI numbers were just boosted noise, not real useable signal. That is what is confusing you. Part of that was boosted noise.

The BCD325P2 has a neat spectrum analyzer function which I constantly use. You press the Hold button, then press the Function button and then keypad #3. And you will see your signal. Analog and even digital trunked signals, i.e. it will usually land on the control channel. Use that too in your antenna adjustments! I always use this particular scanner function when I'm after the antenna unicorn.

 

[paulmohr]

So the RSSI numbers are about what I thought, looks pretty but isn't really useful information.

I tried that spectrum analyzer function once, didn't know what any of the information meant so it didn't do me much good lol. Could someone link me to some info about what it means and how it works?

Like you guys have told me, I am really just chasing ghosts here and I might as well give up. From the way it sounds even if I dropped a bunch of money into a special antenna, dedicated scanner and risked my life mounting it I probably still would not get it to do what I want. There is just no reasonable way for me to pick up a 800mhz tower site that is 40 some miles away two counties over. The hard truth is they designed to not broadcast that far, period. I am not going to trick it into working, at least not easily or affordably.

The closest thing I can compare it to in my mind would be audio frequencies. I am into home theatre and audio equipment, like building my own speakers and stuff. So this would kind of be like intentionally adding a crossover or notch filter to a speaker to filter out a certain frequency or range. Then trying to hear that signal by boosting it with an EQ and/or turning up the volume real high. I might be able to hear it eventually, but it is going to completely throw off the balance of the rest of the system, or limit me to a vary narrow frequency range. And it will probably have a ton of background noise and sound like crap anyway. Why would I intentionally try to filter out, or tone down a frequency and then try to trick my amp into reproducing it anyway.

And if someone were to say 'Hey, I can't hear any sound below 40 hertz, can you make that part louder?" The answer would be, No, I designed it that way sorry about your luck.

In the end these are not really "our" signals to listen to, so I guess we work with what have and be happy with it.

Is that pretty much the Gist of it?

I took a drive to Toledo today and took the scanner along to see how it would do on the move.
Surprisingly well actually. As I got into range of the specific site towers I had no problem picking stuff up. And I didn't have to be as close as I thought, that little cheap antenna keeps on amazing me lol. I drove through Lenawee (adrian to blissfield on out to the 23 highway exchange. I had no trouble picking up Lenawee and Monroe county. Then I got on 23 heading south and into Sylvania, toledo and maumee (lucas county). Very quickly I started picking up sylvania, toledo and maumee police. I was even getting fulton county as well. While I was I sitting in a parking lot waiting for my mom to shop I had to actually lock out some detriot analog signals, and Henry county. From Maumee ohio to detriot on a little six inch antenna is pretty impressive to me! This thing is STELLAR on 155mhz.

I left maumee and worked my way through holland, swanton and delta and back up towards adrian michigan. I had no trouble picking up these towns, including wauseon and napoleon which are farther to the south west. Once started up route 109 and got out of delta heading towards adrian I starting picking up Lenawee sheriff again. I am pretty impressed with how well this little thing works. I would bet with a mobile antenna and a gps with the right programing it would work great driving from city to city. As it is I have each county on a quick key so I can turn them on and off as I change areas. But most of the time I just left it scanning all 1200 some channels and it worked pretty well.

I didn't really want it as a mobile scanner, I do more sitting at home listening, but it is nice to know it will work that way if I wanted to do it. I bet if I wanted to go on a cross country trip I could get a gps, spend about an hour programming my route and I would be able to pick up everything from here to california as I drove along.

Thanks for all your help and input. Know I just need to learn how that analyzer thing works hehe.

 

[RF-Guy-El-Loco]

Go to Mark's Scanners. They have the manuals online. Easier to Read BCD325P2 Digital Scanner Manual

The Bandscope mode just gives you a visual indication of relative RF signal level with each sweep. The higher the signal bar, the better. You get a small RF frequency range to "visually see". You have a Start freq and a range End frequency. This is range you see. You have adjustments for resolution which affects that frequency range and how also long it takes to do the full range sweep. If you press the .No key, the adjustment parameters (freq span, center freq, step, etc) will blink and you can change these values to suit you. The frequency that you stopped on "Hold" will show up as the Center Freq and be labelled with a marker (MF). If you press the Hold key it will give you the actual audio of that signal too! If you twist the MFC knob on top around, you can move the locator pointer to a different freq. Bandscope is awesome!

When you get a very low or nonexistant digital trunking control channel signal bar, you will have major problems monitoring that system. You have to see a small bit of bar (the signal itself) for the scanner to monitor that system. For a distant APCO P-25 system, you may see no trunking control channel signal bar at all appear, and the scanner will probably not receive that system very good. That is the key to hearing that system. No bar, No hear. It will show nearby signals giving you a clue as to interference. Try it first on say a weather channel, see how it works with different antennas you have. This is fabulous feature, try to figure it out as you will need it. It is a basic version of a professional RF spectrum analyzer. With a Yagi antenna, you will use this bandscope mode to peak it in for the highest signal level bar on the control channel.

You can stop on your fav channel or punch in a freq, but if there is no activity on that frequency, no bar will be seen. There are highly detailed adjustments you can make, but Mark's will give you the info. Bandscope does not work like Close Call at all, two different things.

 

[krokus]

There is just no reasonable way for me to pick up a 800mhz tower site that is 40 some miles away two counties over. The hard truth is they designed to not broadcast that far, period. I am not going to trick it into working, at least not easily or affordably.

Are you trying to pick up the Downriver system? Detroit branch of MPSCS?

40 Miles is a tough bit of a challenge on those frequencies. If what you want to hear is a wide-area talkgroup, then it is just a matter of someone being affiliated on your local tower.

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