What if I added something between the mast and the dipole assembly to increase the distance between the dipole and the mast on the VHF antenna? I have a Cushcraft antenna that looks like a VHF version of a DB-404. The dipoles are a little shoter tip to tip than the ones on a DB-224a but the SWR is good all the way down to 146mhz. The major difference I see is how far from the mast the dipoles are.
----- Original Message ----- From: [email protected] <[email protected]> To: [email protected] <[email protected]> Sent: Mon Aug 25 13:21:44 2008 Subject: [Spam] Re: [Repeater-Builder] UHF Repeater Antennas (combining threads) Thanks Jim -- I have been considering lengthening in such a similar fashion. Did you have any noticeable pattern distortion problems or any other side-effects? On Aug 25, 2008, at 10:36 AM, Jim Brown wrote: I have had good luck modifying VHF antennas cut for the 155 mHz band down into the ham band by adding a short stub to each end of each dipole. I flatten a piece of an old TV antenna that has a round element that is rolled into a tube and put a screw through the flattened end wrapped around the end of each element. After the extension is installed I cut the extension to 2 inches. I don't see why the same idea can't be carried over to the UHF antennas. I would suggest a simple way of adding some length to a 440 element would be to drill a hole through the element on each end and put a brass screw through the hole. You can adjust the length of the extension to center the antenna down in the ham band. I did not modify the harness on my 2 meter antenna conversions, and I doubt you would have to on the 440 antenna. When I modified the VHF dipoles, I removed one dipole and connected my SWR meter to the dipole terminals and found the center frequency. It turned out to be 155 m Hz in my case. I then experimented with the extensions until I got the dipole down to 146 mHz and found the extension to be 2 inches. I would suggest that you do the same, and when you find the extension for the one dipole to get it down to 445 mHz, add the same length screw to each of the other dipole ends. Using a #8 or #10 screw might do the trick. I would allow it to stick through the dipole at least an inch, and see where the center frequency of the dipole winds up and adjust the length to center the frequency where you want it. The dipole may not be 50 ohm, as I recall my VHF dipoles were showing about 100 ohms at resonance, so just move the SWR minimum to where you want it by adjusting the screw length and reattach the feedline harness. 73 - Jim W5ZIT --- On Sun, 8/24/08, Cort Buffington <[EMAIL PROTECTED]>< / i> wrote: From: Cort Buffington <[EMAIL PROTECTED]> Subject: [Repeater-Builder] UHF Repeater Antennas (combining threads) To: [email protected] Date: Sunday, August 24, 2008, 4:30 PM Folks, I have a similar question about UHF Amateur repeater antennas so I'll try to combine threads if possible. I just tested my two DB-420s with freshly re-taped harnesses and cleaned up connections, etc. I'm getting somewhere around 1.7:1 to 1.8:1 VSWR on 444.825, the repeater transmit frequency. Here are my ponderings to the combined wisdom of the group (religious and highly opinionated advice welcomed). The advice here is ALWAYS helpful, so please folks, don't be bashful :) 1) Just use it as is, after I put the 105' of hardline on it and all that'll be "good enough"? 2) Try some sort of modification to improve performance in the ham band? 3) Ah, forget the 20' tall antenna, just put up a Diamond X50 and call it good? 4) Give up repeater-ing 73 DE N0MJS P.S. repeater location is the top spot on a 100' tower in a rural location that is relatively high terrain. The repeater is meant to cover between two cities 25 miles apart, a n d is between them. Transmitter is a 100W TPL RXRF that I'd like to run at the 100W. I have remote receive locations and will place remotes and a voter if I can "talk loud enough" to warrant it. -- Cort Buffington H: +1-785-838-3034 M: +1-785-865-7206 -- Cort Buffington H: +1-785-838-3034 M: +1-785-865-7206
