My observation with VHF stationmaster type antennas is if they don't have a support strut, wind motion eventually causes the base decoupling network to crack which, in turn, causes a discontinuity that creates noise. The antenna can be disassembled and re-welded but eventually the problem will return.
If you have a top-mounted Stationmaster there's no way to add a center brace for stabilization purposes. For those that like to save money and experiment a bit, one solution is to replace the damaged innards with a gain J-Pole inside of the radome. The one I constructed consisted of a length of #12 wire with a phasing coil and another length of #12 wire connected as per the standard J-Pole feed assembly. As I recall, the overall length was around 16 feet. Since the orginal radome is on the order of 22 feet, the gain J-Pole was hung inside with a light rope tied to the copper tip and spaced such the RG-58 UHF male fitting from the J-Pole matching point was accessible just inside the antenna base. A popular antenna modeling software indicated the gain was only slightly less than the claimed gain for the original Stationmaster. The radiation pattern was identical. Now comes the tricky part. The Q is higher than that of the Stationmaster and hence the bandwidth is less. If the antenna is initially matched on something other than its support structure the SWR at the target frequency will most likely be higher than desired. Also the antenna's radiation pattern may be altered due to feed line coupling. There may be alternate solutions for resolving these problems but the technique I used was to insert a Tee in the feed line near the base of the antenna with a short coaxial stub. Done properly this lets the transmitter 'see' a 50 Ohm load, isolates the feedline from re-radiation and provides a DC short for lightning protection. The secret of this approach is in knowing where to insert the Tee and how long to make the shorted coaxial stub. 73 de Jack - N7OO ----- Original Message ----- From: mch To: [email protected] Sent: Thursday, April 19, 2007 12:09 PM Subject: Re: [Repeater-Builder] Re: Price On A Station Master VHF Repeater Antenna My experience with MANY Stationmaster type antennas for VHF is that they develop duplex noise problems. Save yourself a lot of trouble and get the dipole type antenna. As a bonus, the dipole type has a full 6.0 dB gain, and not just 4.5 dB as the 2M Stationmaster does. They have to derate it since the elements are too long to fit in the standard radome. Joe M. Tony L. wrote: > > The equivalant Telewave lists for the same price, $968. I can't recall > what the Telewave ham club discount is though. Anything greater than > 30% will bring you in lower than the Stationmaster. > > FYI though, there have been many posts here over the years suggesting > that fiberglass coated antennas aren't as suited for repeater operation > as are exposed dipole arrays. You might want to read some of those > comments before locking yourself in. > > --- In [email protected], Steve Kometz <[EMAIL PROTECTED]> > wrote: > > > > You might check with Telewave. > > They still discount for Ham clubs if that applies. > > > > > Yahoo! Groups Links > > >
