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Why are notch filters used on the transmit side of a repeater duplexer?

The primary purpose of a duplexer is to keep the transmitter output out of the receiver input, since they both operate at the same time from a common antenna on frequencies which are closely spaced. Secondarily, (say, at a hilltop radio site where a bunch of transmitters are clustered) it keeps strong signals from neighboring transmitters out of both the receiver and transmitter, so the receiver doesn't desense and the transmitter doesn't suffer/produce IMD.

The secondary purpose is obviously accomplished with bandpass cavities in line with both the transmitter and the receiver, and the primary purpose is accomplished by also adding notch filter cavities. My question is: Why are there always notch filters (on the receiver frequency) shown in line with the transmitter output?

Obviously, you want notch filters (on the transmitter frequency) in line with the receiver to keep the transmitter output out of the receiver input. But you don't want notches (on the transmit frequency!) in line with the transmitter output, because that would block the transmitter from the antenna. And if they're tuned to the receiver frequency, what exactly are they accomplishing in the transmit circuit? The transmitter doesn't need protection from the receiver, because the receiver doesn't transmit any signal.

It seems like it would be better served by placing those notch filters on the receive side to further attenuate the transmit energy, or else spend the money on bandpass cavities instead of notch cavities to better isolate the repeater from everyone else on the hilltop.

And yet, every duplexer diagram or setup I've seen seems to have notches between the transmitter and the antenna (usually the same number as on the receive side). Am I missing something here?

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    $\begingroup$ Hello and welcome to ham.stackexchange.com! $\endgroup$ – rclocher3 Oct 26 at 19:21
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    $\begingroup$ hi! I've never seen a transmitter with a notch cavity filter, but I've also never built a transceiver with cavity filters for ham purposes. Just out of interest: could you point me to one of these designs? $\endgroup$ – Marcus Müller Oct 27 at 12:01
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    $\begingroup$ The transmitter could (and probably does) emit signals that could well be on the receive frequency, so a notch filter on the transmitter tuned to the receiver frequency would be an added measure of isolation between the two. Remember that if the transmitter is putting out 100W, then 60db of attenuation is 0.1mW. That doesn't sound like a lot, but when fed directly into the receiver would cause all kinds of problems. $\endgroup$ – Duston Oct 27 at 12:58
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    $\begingroup$ @MarcusMüller This is particularly about duplexers used in amateur repeaters, not transceivers, transmitters, or receivers per se. The transmitters and receivers I work on at the day job use cavity filters, but they're much simpler than duplexer cavities. I don't want to call anyone out specifically about their web pages, but pretty much any page turned up via google about "repeater duplexers" has the same TX-BR-BP-Ant-BP-BR-RX configuration, mainly varying in the number and order of BP and BR cavities on each side. $\endgroup$ – AC0CW Oct 27 at 17:39
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    $\begingroup$ Shouldn't some of these comments be answers? They look good enough to me to be. $\endgroup$ – Mike Waters Oct 27 at 21:25
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The transmitter doesn't need protection from the receiver, because the receiver doesn't transmit any signal.

Indeed. The notch filter in the transmit side is instead to protect the receiver from the transmitter.

As well as the familiar harmonics and spur signals generated on specific frequencies by the transmitter it also produces broadband noise centred on the transmit frequency and extending out far enough to cover the receive frequency.

If the broadband noise on the receive frequency was allowed to enter the receiver it would desense (i.e. desensitise) it; incoming receive signals would need to be strong enough to exceed the transmitter broadband noise level.

We get rid of this noise (or at least reduce it below the receiver's own noise floor) by putting a notch filter tuned to the receive frequency in the transmit path. Note we can't put this filter in the receive path as it would filter out the desired incoming receive signals.

Some more reading:

Transmitter noise/receiver desense primer

Transmitter Broadband Noise (shows calculations)

Or try your preferred search engine.

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The transmitter doesn't need protection from the receiver, because the receiver doesn't transmit any signal.

Perhaps not, but consider:

  1. The transmitter makes broadband noise and intermodulation products that may fall on the receive frequency. More isolation is better. And,
  2. The transmitter should ideally also not load the antenna at the receive frequency. If you imagined the transmitter as an ideal voltage source, this would reduce the available signal at the receiver to zero, were it not for the notch filters!
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  • $\begingroup$ Ah, #2 is a good reason! The best reason I could think of for putting it on the TX side instead of one extra notch on the RX side was "because we like to put equal numbers of cans on each side"... but it does make sense to make the impedance of the TX side at the RX frequency as high as possible to avoid sensitivity loss. $\endgroup$ – hobbs - KC2G Oct 29 at 4:15

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