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  Vertical versus horizontal antenna separation
By Mike Morris WA6ILQ
Graphs at the bottom donated by Kevin Custer W3KKC
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Repeater performance is defined more by RF performance than by audio performance.   You can have the best sounding repeater on the planet, but if the user community can't get into it or hear it then it's worthless.   Audio performance is covered elsewhere on this web site.   Good RF perfomance is a balance between receiver and transmitter... you want neither an "elephant system" (all ears and no mouth) or an "alligator system" (all mouth and no ears).   The first step in optimizing the RF performance is to maximize the isolation of the receiver from the transmitter.

There are some additional thoughs on isolation in this article: Some thoughts on Repeater Receiver-to-Transmitter Isolation.

Isolation is a cumulative number, provided by the collective hardware that comprises a repeater.   Individual units can have good and bad features, some of which can enhance the total, some are neutral, some can degrade the total. For example, a braided feedline instead of a solid corrugated feedline. RF jumpers with a loose weave braid versus a 100% shield. An exciter with sharply tuned interstage filtering to cut down on the off-frequency broadband noise. Or in the receiver a helical front end, or a sharply tuned front end instead of a lumped LC front end. A preamp with helicals versus a lumped gain untuned preamp. A 6-cavity pass-notch duplexer instead of a 4-cavity notch-only model. A notch cavity between the duplexer and the receiver to reduce the transmit signal. And the list goes on....

Some systems run two antennas, some by choice, some by necessity. For example, one local site has a single 403-512 MHz receive antenna at the top level of a 120 foot tower, with a preamp mounted next to it. Inside the building there is an AngleLinear multicoupler that feeds the receivers of over three dozen UHF repeater systems ranging from 413 to 511 MHz (the common name for this type of system is a "community receive antenna"). The transmit side of each group of five repeaters feeds a five-port Sinclair transmitter combiner which feeds a single transmit antenna. All of the transmit antennas are mounted to crossmembers at the 100 foot, 80 foot and 60 foot levels on the tower.

The isolation graphs below date from the late 1950s and early 1960s but isolation derived fron receive-to-transmit antenna spacing does not change, it's simple physics.

Do note however, that the size of the antenna needs to be factored into the system... picture a situation where you have a pair of DB208 high band antennas, each of which is over 40 feet long, mounted one above the other on a tower with six feet of space between them.   Despite what the graph below says, six feet between a receive element and a transmit element outweighs the 40 feet of separation between the radiation centers... don't expect the over 55 dB of isolation between them that the 40 feet on the graphs below would imply...

The curves below show a maximum of 80 dB for vertical separation and 90 dB for horizontal. Do not assume that antenna separation alone will provide a good performing repeater unless you run split site. If done properly, the separation is additive. As long as you don't have one antenna in the major lobe of the other you could add a 50 dB of vertical isolation and 15 dB of horizontal and get 65 dB total. In practice, I've found a good null directly in line with (i.e. under and over) the transmit antenna, and moving the recieve antenna out of the null can result in less isolation.
Note that nulls are environment dependent - if someone else moves something on the tower that the null that you carefully positioned your antenna into may move...

The "Measured Between Antenna Centers" in the subheading should read "Measured Between Antenna Radiation Centers".   The "physical centers" phrase in the second line of the text above should be read as "the radiation centers".   In some antenna designs those locations are not the same.   Also the last part of the last sentence would be clearer if it read "because of coupling from the transmit antenna to the tower and then from the tower to the receive antenna."

Yes, it would take a half-mile to get 60 dB at 6 meters, and twice that to get 6 dB more.   This is why split-site low band repeaters with cross-links are common.

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This web page originally created, posted and is © Copyright June 2000 by Kevin Custer W3KKC.
Separation graphs provided by Kevin Custer W3KKC.
Text, hand-coded HTML and layout © Copyright 2003 and date of last update by Mike Morris WA6ILQ

This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.