OT SWR & radio antennas

[Dogoth]

I tried searching to no avail and I just didn't want to join an amature radio forum to answer one question.

I've seen helical antenna's used for wireless mic recievers. My understanding is that they produce a figure 8 pick up pattern and in turn add "effective gain" to the mics within that pattern. They also reject outside garbage NOT within that pattern giving the mic a better RF S/N ratio (OK it's not TOTALY OT :-).

I'm an amature railfan and have a portable scanner that is programed to the N American rail frequencies (VHF between 159 & 161 Mhz). I know how important SWR is to a transmitting antenna. Doesn't the same hold true for recieving antena's? In other words if I were to buy an antenna cut to the proper length (1/4, 5/8, 1/2 or full wave) of the center of the frequency band I'm trying to recieve, wouldn't it be more efficient at picking up that frequency?

Also does anyone know of a PORTABLE helical or other high gain antenna (possibly one with a magnet mount that I can aim) specificaly made to recieve this bandwidth?

Thanks for any help

[Craig Vecchione]

Would this help?

http://www.dpdproductions.com/page_traintenna.html#atcsmobile

[Dogoth]

Yes. I've looked at similar products. Since these are both transmission & recieving antennas, when they state Xdb gain, do they mean effective transmission gain or reception gain? I realize you can get inline RF amplifiers but they amplify the noise as well as the signal. It seems to me an aimable directional antenna would reject some of this noise. Also, wouldn't a well tuned antenna which resonates at the desired frequency, also nullify (in comparison) other spurious frequencies that sneek by the filters in the input side of your reciever in effect acting as a first stage filter? I don't expect miracles from an inexpensive HH scanner, I'm just trying to make it better.

Thanks for looking it up.