T&T: VHF Antenna Question

[Albin43SDtr]

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 don't know the tuned characteristics of the two antennas you were 
working with, BUT.... antennas are tuned for an "optimum" frequency. 
This means that your antenna may work better on one channel/frequency 
than another. Higher gain antennas normally have a "sharper" 
"perfect/peak resonance". The Marine VHF spectrum is 156.050 - 
162.025 MHz. The NOAA weather channels are 162.550 - 162.525 MHz. 
This, at first glance, is a narrow band of frequencies, but to a 
sharply tuned circuit, it is huge, so the antenna can be constructed 
in a manner where the most critical frequencies are closest to the 
"sweet spot" (frequency at which the gain is highest), or tuned 
simply to the middle of the desired band of frequencies.

These frequencies are from: http://www.navcen.uscg.gov/marcomms/vhf.htm

I am not surprised that a lower gain antenna will pick up an 
essentially off-band/off frequency station (NOAA) better than a high 
gain antenna - a low gain antenna's bandwidth is normally spread out more.

Most VHF low-band (such as citizens' band) whip antennas are tunable 
by simply adjusting the antenna whip as little as 1/32" to see a 
noticeable difference on an SWR meter. For example, I always tuned 
the antenna so that CH 19 was sitting in the "sweet spot". The higher 
the frequency, the more critical it becomes. Some marine VHF antennas 
are similarly tuneable, but I have not tuned any of them, just 
installed and used them on my old sailboat.

The above reflects my experience....I don't claim to be an RF expert. YMMV


Take care and be safe.

Wayne
M/V Celestial
Albin43 Sundeck