Effects of two or more Antennas mounted on the same mast

[kramden66]

Was wondering what opinions i can get on this subject , example if you have a discone scanner antenna mounted on the same mast with a tv antenna or two tv antennas would the fact that those tv antennas being so close have an effect on the discone ? perhaps making it receive more from the direction the tv antennas are pointed ?
I ask because not long ago i was running a scanner on the built in antenna , near it was a tv antenna the type with the knob that changes the length of the rods artificially in 12 different positions , it was not hooked to the scanner but i found by accident if i rotated the knob on that tv antenna it made the noaa weather weaker or stronger , also when a fire call came in it had an effect.
So if there are other antennas on that same mast up on the roof could it benefit or hurt the discone ?

 

[kf9iz]

Let me address your indoor antennas you had experience with first.
The orientation of the antennas to one another caused reflectivity. The selector switch on those indoor "Rabbit Ears" is some type of capacitive or inductive variation to, just as you guessed, caused the antenna to functionally have a favorable standing wave ratio to create a favorable match to frequency.

Having receive antennas on the same mast is fine as long as they are not so close as to create a parasitic element that reflects the radio wave.
There are some technical wavelength formulas involved but since you are not actually dealing with a specific band but rather a broad-banded situation we won't go into those.

Get some separation vertically up the mast. I imagine the TV antennas will be horizontally polarized and the scanner antenna vertically.
Therefore keep the TV antennas below the scanner antenna.

Other than that you should be good to go.

 

[kramden66]

i wasn't worried about the tv antennas interfering causing loss but thinking more if they could cause a little gain it wouldn't hurt.
This also brings to mind from 85 to 95 i had a mast with antennas , i was into cb in those days but wasn't allowed to put up my 5/8 wave antenna , so i took a trunk mount cb antenna attached it to a metal bracket and put it at the top of the mast , the mast was a 10 foot on a tripod , below it was a vhf/uhf tv antenna , below that was two other uhf antennas , i didn't think the cb antenna would work right but i had proper swr , i guess all that other stuff on the mast helped with the ground plane , eventually a scanner antenna was added too , imagine all this on one mast lol , i don't recall where i had the discone on the mast , it was below everything else i think but everything worked great , i heard everywhere on the scanner , this is why i asked if it actually could benefit reception instead of hurting it , i could try added the uhf yagi and see what it does and it if makes things worse remove it.