I've been watching this thread, and did contribute early in the discussion.
All the trials and tribulations (which nearly all sound valid) are why I simply went with the comet 6m/10m diplexer. It has great bandwidth, essentially passing the bandwidth of connected antennas, in frequency sensitative rather then dependent on the impedence of the antenna, and is plug & play. Of course, you still have to get the antennas tuned and working, but you have to do that anyway. If I was retired, or had lots of time on my hands, attempting to roll your own with the motorola method of specific coax lengths would be fun and probably cheaper. The comet diplexer cost about $60. ------ Original Message ------ Received: Sat, 12 Sep 2009 03:38:23 PM PDT From: "w6jk" <[email protected]> To: [email protected] Subject: [Repeater-Builder] Re: Diplex antenna installation using coaxial cable for 10M and 6 M > The problem with this is it's not the way hams usually do things, so it's counter intuitive. The coax is acting as a transformer, not a stub. Worse yet, its purpose is to mismatch the antenna, not to match it. Specifically, you want the off-frequency antenna to present as high an impedance as possible, so that when it's connected across the active antenna the resulting impedance is as close as possible to 50 ohms. Then, when you switch bands, the roles are reversed, so to speak. The optimal cable lengths may well not be a simple fraction of a wavelength. Somebody probably used a Smith chart to work it out. A clever bit of engineering, that. > > Jeff W6JK > > --- In [email protected], Steven Schultz <steven.schult...@...> wrote: > > > > I did take into account the velocity factor in the simulations. > > In Ansoft Designer I can specify physical length and VF. > > In the antenna simulator I am currently using I cannot specify the VF so VF > > = 1 and > > I use coax lengths equal to free space wavelengths. > > > > The document was given to me in PDF format and I can pass it along. > > How do I go about doing so? > > > > After a brief discussion with my friend on this matter he recommended I > > check into > > Repeater Builder and that is what led me here. > > I looked thru the site's files but could not find the document or any > > mention of the subject. > > > > Thanks > > > > Steve > > > > On Fri, Sep 11, 2009 at 8:32 PM, Joe <k1ike_m...@...> wrote: > > > > > > > > > > > The average coax cable of 1985 vintage probably had a velocity factor of > > > 66%. If you didn't figure this into your calculations the coax would > > > appear to be about 1/4 physical length, but would be an electrical 1/2 > > > wavelength. Did you use a velocity factor in your calculations? > > > > > > Would it be possible to scan the Motorola document that you have and > > > post it to the group? I've heard of it but I've never seen it. > > > > > > 73, Joe, K1ike > > > > > > Steven Schultz wrote: > > > > > > > > I will review my simulations and see if match bandwidth is adversely > > > > affected by the 1/2 wavelength transmission line. I will try to > > > > include cable properties. > > > > > > > > > > > > > >
