T&T: VHF Antenna Question

[Gary Bell]

Milt and all,

>>.....We had identical results: the 6 DB gain antenna outperformed 
>>the taller 9 DB gain antenna by 50%, as judged by bringing in a weak 
>>station--we used a NOAA weather station......
>  
>
I haven't heard mention of directions in our discussion.  Do you get 
similar behavior with a distant station from some other direction?  Have 
you any information about your problem persisting when the boat is 
turned at the dock or moved out of the marina and away from all the 
other antennas, boats, structures, etc. there?  I am mindful of the role 
of so called passive reflector and director elements in a beam antenna, 
could your two antennas be reacting with each other, or with other stuff 
in the marina? 

VHF is notorious for multipath propagation, and bewilderingly complex 
reflections and reactions from all around.  Didja ever notice pronounced 
changes in your FM car radio signal as you pulled ahead only a few feet 
in stop-n-go heavy traffic?  That's multipath, and the commercial FM 
radio frequencies aren't all that different from our marine voice VHF. 

I liked the suggestion to rotate one stick at a time to a horizontal 
position for further SWR and remote station testing, keeping in mind 
that the donut or pancake patterns also rotate with the antenna, so the 
horizontal one will only give you information about the other's 
performance with the horizontal one out of the picture. 

I would particularly suggest testing from another location, preferrably 
out on the water.  There you can check out the 9dB flat pancake 
pattern's behavior when your boat rolls and pitches, or if your bow 
comes up much when planing.   Of course big open water provides the 
perfect place to sketch out the directional sensitivity of all your 
radios and antennas in real conditions by noting the signal strength 
from a couple of distant stations on different courses.  Bring your 
technician along with/without another boat for some transmit power tests. 

I also liked the narrow bandwidth 9dB antenna tuning notion a lot...

Keep us posted,
Gary Bell

PS, don't forget that dB stands for deci Bell!