T&T: VHF Antenna Question

[Milt Baker]

This is to ask if anyone might have an explanation for something very odd happening with a couple of VHF antennas on my boat.  What's happening mystifies me and my electronics tech--it ain't supposed to happen but it does!

The boat has a Digital Antenna model 532 VW-R 16-foot 10 DB gain antenna that does not function properly.  It is mounted on the cabin top of my Nordhavn 47.  A few feet away is a DA 8-foot 6 DB gain antenna also mounted on the cabin top at the same exact level.  The antennas are mounted so there is no shadowing from one another or any other antenna or structure.  The two antennas feed identical Standard Horizon QUEST VHF radios.  

The problem is that the 6DB gain antenna outperforms the 10 DB gain antenna by a wide margin.  
To confirm this, my dealer and I swapped antenna leads--no matter which radio we use, the 6 
outperforms the 9 by about 50% when bringing in distant stations. 

Based on that, it appeared to me that something had gone wrong with the 10 DB gain antenna, 
so we obtained a brand Digital Antenna model 532 WV-R 9 DB gain antenna and tried it.  The PL-259 
(end fitting) used was the one furnished by the manufacturer with the antenna, and we ran the cable 
straight from the antenna to both radios with no additional cables or connections.  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.

Neither my technician nor I has an explanation for this.  Clearly, the taller, higher-gain antenna 
should outperform the shorter, lower-gain one.  But here we have two cases where that is not 
happening.

I spec'd Digital Antenna brand antennas because I felt they make the best.  Now I am thinking it't 
time to go back to Shakespeare, whom I used for many years--especially so, since I cannot get a
anyone from Digital Antenna on the phone to talk to me about this.  Also, no e-mail response to 
a request for advice left on their website.

Any ideas what's happening here?  My best explanation is some kind of systemic problem with the 
10 DB antennas.

Many thanks for your ideas,

--Milt Baker 
   Nordhavn 47 Bluewater
   954-610-5193