Paul Kelley N1BUG wrote: > Hi Burt,
snip >> You could try using 3/4 wavelength matching pieces to get the extra >> length. That should be equivalent to 1/4 wave but will be more >> sensitive to frequency changes. > > I only need to extend them by about a foot to get ideal spacing > between dipoles, so 1/4 wavelength with .66 VF would be enough. Be careful how you splice. I normally used short pieces of thin wall brass tubing of appropriate size to splice the center conductor. You can get this tubing in various sizes (increments of 1/32in diameter up to 9/16 OD) (K&S brand) at most hobby shops. You can also use a piece of larger tubing to splice the shields. The most important step is the use of heavy adhesive lined shrink tubing to seal and waterproof the joint. > >> Never having seen one of those antennas (the SD2352) up close, I am not >> sure of the harness configuration and how it would compare to the SD214 >> that I am familiar with. > > Total of 8 dipoles. Impedance at end of coax coming from each dipole > approximately 50 ohms. Two dipoles connect to a type N tee, so at > the tee center should be about 25 ohms. From there, 1/4 wavelength > RG-213 to a a factory harness 'Y' splice, coax should transform the > impedance to about 100 ohms, divided by 2 at the Y so we're back to > 50 ohms coming out of there. From there, approximately 66 inches > RG-213 to the center Y splice of the overall harness. This coax > should maintain 50 ohms, divided by two at the center Y splice = 25 > ohms. There is a 1/4 wavelength of some coax coming out of there, > spliced to a length of RG213 running down to the bottom of the mast. > I'm assuming the 1/4 wavelength matching section is 35 ohm coax, but > cannot confirm that. This description may be "clear as mud"... I can > make a diagram of it later if you want. That is exactly what I would have expected ie same as the SRL210 series. BTW the Wireman may have RG-83 35 ohm coax available - at least he had many years ago when I inquired. When I got my stock of RG-63B I had to buy a 500 ft roll at many $$$$, which was only practical since I intended to make many antennas. I probably only used 175 ft of the roll. The high cost is due to the fact that the cable was true Mil-Spec (complete with compliance records) and small demand/production runs. The same would apply to RG-83. > >> Figuring out the failure mode is the most important first step. Then >> you can go from there to possible solutions whether it is harness >> replacement, repair or dig into the dipoles. > > I have decided no matter what I'm not putting it back up as an 8 > dipole bidirectional array so I will take apart the original harness > for inspection. I will also test each of the dipoles on the repeater > individually to check for noise. If I do not find any problems in > either of these processes, then I will have no clue what caused the > problem! Consider the possibility that water has got into the RG-213 and corroded the shield. This would likely give noise when RF is applied but not be particularly sensitive to vibration. Look for green copper shields, it is not environmentally friendly :-) Burt>> > > Paul N1BUG > > > > > ------------------------------------ > > > > Yahoo! Groups Links > > > >
