Jul 152009

The 8.5dBi omni-directionalount antenna has a partially-sealed hole at the base:  In fact, at the base of the antenna there are two threaded holes.  One of the holes is sealed, the other is not.  The one that is not sealed, if you blow in it J, you will feel air come out the bottom of this base where the threads go in that have the nut on it.  The issue is that there is potential for leakage of water through this hole.

One of our customers asked if there is “any harm in filling that threaded hole (sealing it with silicone say) that is currently not sealed, or did the manufacturer do this for a good reason, in which case, what is it…?  If its not necessary, perhaps it should be sealed it like the other one….?”

After looking into this matter, we found that there will be no problem caused by sealing the “extra” hole with silicone.

Below is a response that I made to the customer speculating about possible reasons for the hole:

THEORY #1: The fact that the unsealed hole is threaded may mean that it is for alternate mounting hardware
THEORY#2:   The hole’s purpose may be to allow air to pass as the antenna encounters changes in temperature, which causes air to expand and contract.  There is not any RF technical reason that an antenna cannot be sealed. 
The following notes are germane only if theory#2 is correct:

I’m wondering if there is some way to cover that hole or partially seal it in a way that would prevent the entry of water or salt spray:  For example, if a temporary type of seal could be applied, so that from time to time, the hole could be uncovered in case there is air vacuum or pressure inside the antenna?

I’m also thinking that in environments in which temperature changes are not so extreme, maybe it would not be necessary to have a hole to allow air to pass:  i.e., a swing from hot to freezing, potentially could cause enough air pressure to do some damage; but a swing from a cool night to a hot day probably not enough to damage the antenna – just my thought.

Another thought on this theory:  If not for the whole, the specs for temperature extremes could not be so extreme.

-George Hardesty

 Posted by at 10:39 am Antennas  Add comments

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