I have two antennas in the following way (image attached). Will the low separation between the two receivers degrade the receiving power overtime?
Old Ham guy here, two antennas and the metal post that close will definitely affect your signals. Any conductive material like metal within 20 inches / 50 cm will significantly affect the received signals.
One option to consider, Mount the antennas on each end of a strong angle brace. Then mount the angle beam to the top of your pole. You will get the extra height for the lower antenna and separate the antennas. You already know where your best distance signals come from, so mount the antennas perpendicular to that direction, thus each antenna should have minimal interaction. If you can find a waterproof tape, wrap the antenna connection to keep out water from the cable. Many of us have found out the hard way how important this can be.
You should see a nice improvement immediately by separating the antennas. Have fun with improving the whole system. Make sure you have a good ground on the pipe, as lightning loves high antennas. Have fun improving your system.
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Thanks for the suggestion. Do you mean something like this?
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Close, simpler to put a T shape brace bolted near the pole top, then each antenna at right angles mounted on the ends. You have the separation right. You can go farther apart for slightly better results, but 50 cm seems to be a minimum distance to reduce interactions at these frequencies. UHF (300 MHz to 3000 MHz). When done, you will have a better antenna than I have! I’m too old to be working on towers or antennas on a 45 degree slope roof. Not afraid of the fall, just the sudden stop at the ground. If the fall did not kill me, She who must be obeyed would. Have fun out there!
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Thanks for your time and advice. I will ponder upon this idea and implement it soon.
ah the good old SWMBO appears again 
not so much difference in that around the world 
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She is the keeper of my “Job Jar”.
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As long as she is the keeper and not the dispatcher you’ll be safe 
Here you’ll get a casual reminder once in a while, after that it will be enforced until breaking point. So don’t tell her how the internet access works or she’ll shut you down until the jar is empty
I should be safe on the internet access. She considers that one of “My Jobs”!
Humor: I wonder if his two antennas are mounted on the close mast or on the very nice red/white tower framed perfectly at the bottom of his photo above. The red/white tower would have a few different complications, but would definitely be fun to sort out!
-Dan
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@MC130E , That used to be a mobile tower, but the company has sold it to someone, so it doesn’t have any equipment.
I’m all for putting up that antenna, but I’m a few hundred thousand dollars short. LOL
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I hope you aren’t relying on that mast pole for a ground connection for your antenna and coax! Looks like that end of the pipe used to be in the ground.
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No that’s numerous layers of tape which was put on it about 5 years ago, that’s why it looks all dirty.
So are you getting your antenna ground from a separate connection to the coax somewhere below the mast mounting point?
Yes, there was a ground till the last thunderstorm season but last month I removed it to install a new one later this month.
Can you give me insight on how your antenna is grounded, mine was a makeshift.
My antenna mast is steel EMT electrical pipe [conduit] and it is supported by an apex section of Rohn antenna tower. The mast pipe and tower are electrically connected by a metal throw bolt that holds the mast pipe in place. I then have a 8 foot solid copper ground rod driven into the ground near to the base of the Rohn apex section and they are clamp connected together using #6 gauge stranded copper wire! Continuity is confirmed using a ohm meter. The coax and antenna grounds are connected at the mounting point at the top of the metal conduit.
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the minimum distance is 0.5 wavelengths; the section of the mast between the antennas must be dielectric.
Cell / Mobile phones have seperate antennas for each of following transmit and receive functions:
- Voice
- Data
- WiFi
- Bluetooth
- Mobile Hotspot
Cell / Mobile Phones measure say 5cm x 10cm x 0.5cm. All these antennas are crampped In this small space. What will be clearances between these 5 antennas? 
While engineers will place antennas in close proximity, it will work sub-optimally. It is all about making a comprise between many factors. The first picture in the first post antennas will work. Just saying, they will work better with a decent spacing away from other conductive materials.
Having wider spacing minimizes the interaction. You can test this using two ADSB antennas to separate receivers and see how well the signals from your local area are received as you change the spacing. This does not have to be the same or different frequency,
Or use a single antenna and move the antenna relative to a middle metal mullion or window frame will show the interaction of an antenna and any other metal object.
Have fun and keep learning always. Gene