2 antennas on tower question

[nick223]

We are putting up an antenna tower 60-80 feet on top the the tower we will have a MFB1503 antenna. We will also be putting up a Ferret 8 scanner antenna with a stand off mount about one/two feet off the tower. My question is can these antenna's be mounted side by side on top of the tower with out having interference? Whats the recommended distance that should be between antenna's?

Thanks

 

[mmckenna]

Ideally you'd want a minimum of one quarter wavelength between the antennas, more if you can do it.
Issue is two fold:
1. Having an antenna too close to a transmitting antenna can throw the radiation pattern off, making the TX antenna slightly directional.
2. Depending on the TX power, if the two antennas are too close to each other, the TX energy can couple to the scanner antenna and do two things:
a. Overload the front end of the scanner causing to to lose reception when the other radio is transmitting.
b. If the TX power is high enough, you can damage the front end of the scanner receiver by overloading it with too much RF.


Personally, if it was me, I put the MFB-1503 at the top of the tower where it can do the most good. I'd avoid putting the scanner antenna next to it to prevent any issues. Instead, I'd mount the scanner antenna farther down the side of the tower. A receive antenna spaced off the side of the tower is going to see very little impact from the tower, compared to the reflections a transmitting antenna would see.
Doing that would minimize any overloading of the scanner front end, reduce the chances of damage, and still work very well.

 

[nick223]

Thank you

 

[dlwtrunked]

Ideally you'd want a minimum of one quarter wavelength between the antennas, more if you can do it.
Issue is two fold:
1. Having an antenna too close to a transmitting antenna can throw the radiation pattern off, making the TX antenna slightly directional.
2. Depending on the TX power, if the two antennas are too close to each other, the TX energy can couple to the scanner antenna and do two things:
a. Overload the front end of the scanner causing to to lose reception when the other radio is transmitting.
b. If the TX power is high enough, you can damage the front end of the scanner receiver by overloading it with too much RF.

Although I agree with your advice to him, I would change 1. to say " Having any metal object (including the tower or another antenna) too close to a transmitting antenna can throw the radiation pattern off, making the TX antenna slightly directional."

 

[jonwienke]

It will also increase SWR.

 

[nick223]

Here it is. The picture shows the 2 antennas fairly close but it's just the angel the picture was taken at. The scanner antenna has little reception coming from a distance. I mean it picks up local traffic great but neighboring towns 15-20 miles away it won't even pick up. You'd think being 65 ft in the air it would get good reception. Our old antennacraft ST2 about 35 ft up picked up amazing to bad it broke. Is there something I can check as to why this new scanner antenna basically sucks.

 

[Thunderknight]

The scanner antenna has little reception coming from a distance.

What frequency range are you trying to receive?
What type of coax cable is feeding the antenna?
Is it a whole new run, or did you just extend the old run?
Are you using a scanner as the receiver?

 

[nick223]

It receives 800 range good. But 151-158 VHF at a distance stinks.
All new cable runs
LMr400 to a PolyPhaser then Lmr 240 to the scanner

 

[Thunderknight]

You might be overloading the front end of your receiver.

 

[nick223]

You might be overloading the front end of your receiver.

What you mean? Please explain.

 

[Thunderknight]

What you mean? Please explain.

Some receivers, especially wide coverage receivers (scanners) and those without a lot of front end filtering (many scanners) can become less effective if they have too much signal hitting them (poor selectivity). Some is just plain overload and some is intermod.
Overload - think of trying to look at something in a dark area outside on a bright sunny day
Intermod - think of trying to look at something far away but someone is also aiming a flashlight into the side of your eye.

Receiver front-end characteristics | content content from Urgent Communications

If you have a land mobile radio (e.g. Motorola CDM, XTL, Kenwood TK-790, Harris M7100, etc etc etc) you can try, see if that receives the same frequencies but much better.

At my house I found that my scanners were just routinely being subject to pager intermod and FM broadcast overload on VHF. Even with filters ahead of the scanner, it wasn't perfect. I changed my receivers to some Motorola CDMs and it was like night and day.

 

[dlwtrunked]

Some receivers, especially wide coverage receivers (scanners) and those without a lot of front end filtering (many scanners) can become less effective if they have too much signal hitting them (poor selectivity). Some is just plain overload and some is intermod.
...
At my house I found that my scanners were just routinely being subject to pager intermod and FM broadcast overload on VHF. Even with filters ahead of the scanner, it wasn't perfect. I changed my receivers to some Motorola CDMs and it was like night and day.

I always tell everyone I know with a VHF scanner/receiver to put a cheap FM notch filter in the antenna line even if they think they do not have a problem. Even those who think they did not have a problem have found it works miracles as nearby FM broadcast stations were simply de-sensing the receiver without giving a hint otherwise that they were doing that. In addition, I use paging filters-VHF paging needs to die.

 

[nick223]

The base radio kenwood tk 7180 coming off the other antenna works fine.

I do use a MCA204M
VHF/UHF Receiver Multicoupler - 25 MHz to 1 GHz
For the scanners

 

[nick223]

Notch filer like these?
http://www.stridsberg.com/prod02.htm

 

[mass-man]

Do you have another antenna you can attach the scanner to? if it works OK, that would mean the Ferret has a problem....not likely, but a possibility. I am sure with the new cable run, you checked everything before the install...connections, waterproofing, etc.

I believe what Thunderknight is saying is it is possible the antenna is doing such a good job in the 151-158 that TOO much signal is arriving at your scanner and overloading the front end. Does your scanner have an attenuation button? often seen as an att button....

Is the VHF station you want to hear on the side of the tower with the antenna or is the metal of the tower in the way,..moving the antenna even a few feet could make all the difference..

Keep us updated on what you discover....

 

[mass-man]

appears you were getting several responses at the same time....

Yes that filter would work if it is FM radio stations that are desensing your receiver..

Give this short explanation a read...it could be any number of offending signals....and yes VHF paging is a big problem....
Amateur Filters | PAR Electronics | Filters for the commercial 2 way market, MATV, FM broadcast, laboratory, marine industry, amateur radio, scanner and short wave listening enthusiasts

 

[mm]

The lower antenna should be mounted on a standoff arm/mount that basically places the antenna at minimum 1/2 wavelength out from the tower and it should also place the vertical separation a minimum of 1/2 wavelength in the Z(up/down) direction from the upper antenna.

One wavelength spacing in both the x,y and z directions would be even better than 1/2 wavelength mentioned above.

These spacings will enable the lower antenna to receive in an almost 360 degree azimuth while also minimizing antenna to antenna interference.

Also that is a big waste of tower space, you really need some LOW BAND folded dipole STACKS along with a HF LOG PERIODIC to really make that tower happy.

Remember this: A happy tower will stay up for years whereas an unhappy tower will result in a tangled mess of metal on the ground, always keep your tower happy, the more antennas the meerier.

 

[buddrousa]

Nick 2 things
1. What type antenna is that? Brand and Band?
2. Have you bypassed the multicoupler and went straight to the scanner?

 

[nick223]

The lower antenna should be mounted on a standoff arm/mount that basically places the antenna at minimum 1/2 wavelength out from the tower and it should also place the vertical separation a minimum of 1/2 wavelength in the Z(up/down) direction from the upper antenna.

One wavelength spacing in both the x,y and z directions would be even better than 1/2 wavelength mentioned above.

These spacings will enable the lower antenna to receive in an almost 360 degree azimuth while also minimizing antenna to antenna interference.

Also that is a big waste of tower space, you really need some LOW BAND folded dipole STACKS along with a HF LOG PERIODIC to really make that tower happy.

Remember this: A happy tower will stay up for years whereas an unhappy tower will result in a tangled mess of metal on the ground, always keep your tower happy, the more antennas the meerier.

Like I said previous post the picture makes the antennas appear closer then they actually are. The scanner antenna is on a stand off from the tower probably 1-2 feet off. Plus the wavelength mention above is how the scanner antenna is from the 2way radio antenna.

 

[nick223]

Nick 2 things
1. What type antenna is that? Brand and Band?
2. Have you bypassed the multicoupler and went straight to the scanner?

1 MFB1503 antenna. Ferret 8 scanner antenna

2 not yet just came here for ideas on to where to start troubleshooting