Jsn, The isolation you achieve for a given separation won't vary by the split, only the amount you need.
As others have said, you may manage to get a functional repeater with 20' or less separation. I have no idea where the other 120' estimate is coming from, but it doesn't agree with either common published data or my personal experience at UHF. Just make sure you use good quality, well-shielded feedline. First-time homebrew duplexers and filters rarely work as well as cheap hamfest duplexers, and usually cost more to build. 73, Paul, AE4KR ----- Original Message ----- From: jsn To: [email protected] Sent: Friday, October 31, 2008 9:29 AM Subject: Re: [Repeater-Builder] Re: 2-in-one antenna for UHF repeater? This is encouraging. My second radio hasn't arrived yet, but I will definitely build 2 dipoles 20 feet apart just to see how well it works. I am building a portable GMRS repeater for use in the remote woods. GMRS limits you to a 20 foot antenna, so I could have the Rx at 20 feet, and the Tx at "ground level" which will probably be a vehicle roof most of the time. According to the chart here (http://www.repeater-builder.com/antenna/separation.html) 20 feet would give me 55-60dB of isolation (but the chart doesn't seem to specify the offset, which is weird). What would be considered a minimum usable dB figure for isolation? I will probably only be running 10W or so. John, can you (or anyone listening) provide a pointer to information on the "helical" you mention below? Is it possible to add to the isolation between antennas using some sort of electronic filter that is more easily buildable than a full scale duplexer? Can you do any good with capacitors and toroidal or air core inductors? I'm also not afraid of soldering up a bunch of copper tubing or whatever if that is what it takes :) Thank you all for your help. I appreciate your willingness to help an overt newbie :) I am making my way through the wealth of information on the repeater builder site, which is an amazing resource. jsn On Fri, Oct 31, 2008 at 1:14 AM, jgielis <[EMAIL PROTECTED]> wrote: > I can vouch for this statement.. > Our Club has set up a UHF rptr with 5M vertical separation > using a Philips FM815 (40W) and dual feedlines (LDF4-50), > the other feedline feeds the TX ant. > The only filter is a small helical on the RX. > > 73 John > VK4JKL > > On 31/10/2008 4:58:14 PM, jistabout ([EMAIL PROTECTED]) wrote: >> Jsn, you can do just fine with two good antennas with good quality >> feedline with as little as 10 feet of vertical separation at 5 mhz >> frequency spacing. >> >> A quick look at the chart within the ARRL FM & Repeaters Handbook >> shows that you can achieve 75db of isolation with just 20 feet of >> vertical separation at 450mhz, more than enough for duplex operation. >> >> I personally have built UHF systems which use two antennas with close >> spacing and they all show no significant desense and work just fine. >> >> 20-30
