Running one scanner with multiple antennas

[loumaag]

Okay, enough; Let's get back on topic

Fellows,

Please take the acrimony somewhere else.

Now, everyone, remember, when you are discussing matters here in the forums, you have several levels of audience, please try and steer your technical discussions to the technicians, thanks.

 

[WA1ATA]

Is there a sticky or wiki on intermod and front end overload where the last several posts could be moved to? There is a pretty clean break right at post #61. Everything from #61 onward could be moved from this thread without affecting the main topic.

Post #61 onward could be moved to a new thread (or sticky) since it doesn't rely upon any of the earlier posts. Possible titles of new thread would be "notch filters" or "Preventing front end overload".

Lots of good info, even with a bit of unnecessary back-and-forth.

 

[loumaag]

Thanks for the suggestion!

Indeed I have done that and removed 36 posts to a new sticky topic:
http://forums.radioreference.com/antennas-coax-forum/212129-notch-filters-effects.html

For those who may be confused I left my previous comments (in regard to the 36 posts I moved) and the above suggestion here.

 

[equineguy]

After searching many posts for multiple antennas, decided to pick on this one

Experimenting with my Mobile scanner with two antennas ( one pigtail and one center loaded ), both on the same type of cable and distance ( rg58 ). I have them hooked into a simple radioshack splitter and it picks signals up - could'nt tell you if its better performance or not... but there is something that occurs that I cant explain - maybe someone smarter can explain it in simple terms for me.

Scanning the weather channel with both antennas hooked up I get 162.400 clear. If I disconnect one antenna, it scans to 162.550 and that is now the strongest signal. Re-attach the second antenna and it goes back to 162.400. I did this multiple times and its consistant and its kinda neat - but I dont understand why it does this. Makes me think that the splitter does not allow the combined antennas become more wide range. Being east of 95, I am not considerably closer to either one of the stations.

 

[popnokick]

... the splitter does not allow the combined antennas become more wide range.

Using multiple antennas with a simple "T" type splitter DOES NOT in fact allow the combined antennas to become more "wide range".... and you have just provided a firsthand observation of that fact with your experiment / testing. Instead, what you have seen is the unpredictable nature of enhancement / nulling or attenuation of signals that occurs when you combine two antennas on a "T" connector to a single radio.

 

[equineguy]

This is quite fascinating...

 

[prcguy]

You can experience the same effect with your stereo, which is a more predictable experiment. Swap the polarity on one speaker lead and play some music. Anything that is or was mixed to the center, meaning equal level in right and left will now be severely attenuated. Instruments that were mixed panned to the left or right of center will suffer different levels of attenuation depending on how far from center they were.

The stereo experiment above inserts a fixed 180 deg phase shift on all signals and provides a known result. Connecting a T adapter between two antennas with unknown lengths of coax will give random attenuation on some signals at some frequencies and a possible increase in other signals if they arrive in phase at the T connector.

Bottom line is don't use a T adapter or power divider/splitter to connecto two different antennas. If the antennas are designed for different frequency bands the proper combining device is a diplexer, triplexer or similar, which will combine different freq antennas for different bands with little to no loss.

You can however use a T adapter or backwards splitter on two identical antennas, preferably directional and if they are at a correct spacing you can get up to 3dB gain by doing this. If everything is not perfect you can easily make a big attenuator out of it.
prcguy

Using multiple antennas with a simple "T" type splitter DOES NOT in fact allow the combined antennas to become more "wide range".... and you have just provided a firsthand observation of that fact with your experiment / testing. Instead, what you have seen is the unpredictable nature of enhancement / nulling or attenuation of signals that occurs when you combine two antennas on a "T" connector to a single radio.

 

[talkpair]

To demonstrate prcguy's point without rewiring, I have created a stereo mp3 of a 440 Hz tone where the left side audio is 180 degrees out of phase with the right side audio.

Play the file using external speakers spaced a few feet apart. Cover one ear with your hand and listen with your open ear facing the speakers. Move around the room in the general vicinity of the speakers with the open ear facing the speakers and you will experience radical changes in audio level.

 

[cpd417]

I'm new to scanners and just purchased a Radio Shack Pro 652 and mounted it in my cruiser to monitor neighboring agencies. I've read through here but don't really understand a lot of what is being said. I'm trying to listen to a 150.xxxx (vhf p25) and 453.xxxx (uhf p25). Is it best to use one multi band antenna or can I run two separate ones into a tee?

 

[talkpair]

To simplify things, go with the multiband antenna for now.

Get something with an NMO mount so you have the flexibility in the future to swap antennas out if you want to try something different.

 

[lu81fitter]

So.... I have a mobile installation in my truck. I have a scanner, BCT 15x. I have 2 antennas mounted on the roof. Both NMO mounts drilled in about 16" apart. The coax is whatever length that came with the mounts, I think about 15 feet. One antenna is a VHF 1/4 wave whip (approx 18"), the other is a UHF 1/4 wave whip (approx 6"). The coax from the antennas is fed into a diplexer (1.3 - 170 MHz for the VHF & 350 - 540 MHz for the UHF). Then is routed to scanner. I seem to be able to monitor systems (analog) quite well. Is this installation adequate, or is there room for improvement??
Thanks for any insight.
Please note----->. This is a RECEIVE ONLY install.

 

[Project25_MASTR]

So.... I have a mobile installation in my truck. I have a scanner, BCT 15x. I have 2 antennas mounted on the roof. Both NMO mounts drilled in about 16" apart. The coax is whatever length that came with the mounts, I think about 15 feet. One antenna is a VHF 1/4 wave whip (approx 18"), the other is a UHF 1/4 wave whip (approx 6"). The coax from the antennas is fed into a diplexer (1.3 - 170 MHz for the VHF & 350 - 540 MHz for the UHF). Then is routed to scanner. I seem to be able to monitor systems (analog) quite well. Is this installation adequate, or is there room for improvement??
Thanks for any insight.
Please note----->. This is a RECEIVE ONLY install.

1/4 wave on VHF just so happens to be 3/4 wave on UHF (so it's naturally resonant ). You'd see nearly identical performance minus the diplexer and secondary antenna on a receive only setup.

Sent from my SM-T350 using Tapatalk

 

[prcguy]

The VHF antenna being 3/4 wavelength on UHF will provide a reasonable match, but the radiation pattern will be an upward pattern with less signal at the horizon than a 1/4 wave resonant UHF antenna.

If the OP wants to improve reception the separate mounts and diplexer will allow him to use maybe a 5/8 wave on VHF and a gain type on UHF, or run a gain type VHF/UHF dual band and a separate gain type 800 antenna using a different diplexer.

Or a VHF/UHF combo and a separate VHF lo antenna, etc.
prcguy

1/4 wave on VHF just so happens to be 3/4 wave on UHF (so it's naturally resonant ). You'd see nearly identical performance minus the diplexer and secondary antenna on a receive only setup.

Sent from my SM-T350 using Tapatalk

 

[lu81fitter]

W5PKY, before adding the UHF whip, I was receiving a department on 453.55 OK. With the introduction of the additional antenna, it comes in clearer.
Prcguy, I do, in fact, have a 5/8 wave VHF antenna, and a 5/8 over 5/8 gain antenna for UHF. Those make it difficult to go in and out of the garage, so I usually just use the 1/4 waves unless I travel farther away and still want to receive distant signals.
I am considering using a triplexer for the addition of an 800 MHz antenna as well.
BTW, the frequencies I monitor now are between 145 - 160 MHz and 445 - 465 MHz. No VHF low.

 

[Project25_MASTR]

W5PKY, before adding the UHF whip, I was receiving a department on 453.55 OK. With the introduction of the additional antenna, it comes in clearer.
Prcguy, I do, in fact, have a 5/8 wave VHF antenna, and a 5/8 over 5/8 gain antenna for UHF. Those make it difficult to go in and out of the garage, so I usually just use the 1/4 waves unless I travel farther away and still want to receive distant signals.
I am considering using a triplexer for the addition of an 800 MHz antenna as well.
BTW, the frequencies I monitor now are between 145 - 160 MHz and 445 - 465 MHz. No VHF low.

Interesting. I've generally never had a problem with using 1/4 wave VHF to monitor everything through 900. You could also try a muliband antenna paired with an appropriate diplexer covering V/U and 7/800. I've been meaning to find a triplexer for my pickup but haven't had the time.

Sent from my SM-T350 using Tapatalk

 

[lu81fitter]

Quote from W5PKY.
"Interesting. I've generally never had a problem with using 1/4 wave VHF to monitor everything through 900. You could also try a muliband antenna paired with an appropriate diplexer covering V/U and 7/800. I've been meaning to find a triplexer for my pickup but haven't had the time." <End quote

I ordered my diplexer through The Antenna Farm. It is a CF-4160J. As I said, it seems to work well. My antenna connections were the BNC type, so I had to order the appropriate adapter as well. They have other diplexer and triplexer options there. Just a thought if you do look.
Also, the system I monitor on UHF is about 25-30 miles away (as the crow flies), and I live in a bit of a valley, so reception was weak. I wanted to give the diplexer thing a try anyway just to see how it did.
I also have an antenna from TRAM that is multi-band. (Model # 1182). It offers improved performance over the 1/4 waves, but truck will not fit in garage with any of the gain antennas I have, hence my quest for better reception with the shorter antennas. Taking antennas on and off is a pain, especially if the weather is bad.

 

[AZDon]

Triplexer

Getting back to the beginning of this thread has anyone actually measured the difference between using a triplexer with 3 different band specific antennas vs a single band specific antennan?

No longer have access to a service monitor but I can find no perceivable difference betweeen using a triplexer or inserting one of three antennas VHF Hi, UHF and 700 MHZ to the same LMR 400 50" cable. All I have to measure the receieved signal is an S meter on several differant radios, results, no loss with the Triplexer.

 

[iMONITOR]

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