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Re: VHF MTR2000 Poor Receive Performance
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Yes, if those are the factory supplied cables (and Tees) and the desired frequency is within the factory specified range then something else is likely the issue. But if, as is common on this group, he's trying to use a set of cavities made for public service down in the ham band then the cables are probably incorrect. I haven't been paying 100% attention to everything said in this thread. I was generally trying to share some of my own understanding and experience, when making a set of cables for a set of used cavities which did not come with them, with those who might want it. If anything I said was incorrect please illuminate us all. On Monday, October 31, 2022, 11:29:11 AM CDT, Teton Amateur Radio Repeater Association (TARRA) <tarra@...> wrote: Sounds to me like you don't know what you are talking about with this duplexer. Leave the cables alone. This duplexer is designed for the cables to be used for the full frequency range specified for this duplexer. Mick - W7CAT ----- Original Message ----- From: "Bradley Andrews via groups.io" Sent: Monday, October 31, 2022 10:12:44 AM Subject: Re: [repeater-builder] VHF MTR2000 Poor Receive Performance > > Sounds to me like your inter-cavity cables aren't close enough to the right length. > > You probably already know this but I'll state it for those who might not: > The reason the cable length is pretty critical for proper performance is the impedance transformation property of a 1/4 wavelength line. OK that's the general statement but here's the detail: > > If you look at an individual cavity on a VNA, you'll see the expected 50 ohm sweet spot at the pass frequency. The RF flowing in the 50 ohm cable passes through with no reflection. But at the notch frequency, it will be very close to a perfect short or a perfect open. (Which it is depends on the design of the notch mechanism on the cavity and whether the notch is above or below the pass freq.) > > For the sake of our example, let's assume that your cavity exhibits a short at the notch freq. > > That 1/4 wave cable between cavities transforms that short so that at the other cavity it looks like almost an open circuit. But the other cavity is also presenting a short-circuit impedance - being driven by the open circuit impedance at the cable. So the mismatch is about as perfect as you can get (or you can also say, the match is about as imperfect as you can get) and very, very little energy gets transferred. That's why a set of cavities with the correct cable lengths shows a greater notch depth than the sum of the individual cavities. Those additional mismatches contribute significantly to the notch depth. > > Back to your case: The approx. 90 dB notch depth you're measuring is only the sum of the cavities' individual notches. You're not benefitting from the cable mismatches. > > As many have alluded to, unfortunately it's not as easy as just cutting a set of 1/4 wave cables. There is a certain small but significant amount of 'effective length' in the coupling loop, whose effective amount varies with the coupling set and the notch spacing. So your cables are going to be shorter than 1/4 wave from connector to connector - the rest of the 1/4 wave is 'absorbed' in the coupling loop. > > If you're very clever you could probably measure the reactances at the notches with your VNA then calculate how short to cut your cables. That will give you some idea what ballpark you're in. I've tried to do that and either I wasn't quite clever enough or Murphy was having his fun that day (probably both) and I still had to resort to 'empirical' methods to find the optimum cable length. > > I sure learned a lot though! > > 73 > Brad KB9BPF > On Monday, October 31, 2022, 10:42:22 AM CDT, Jared Smudde wrote: > > I went ahead and tried to work on the receive/high pass side. All 3 cavities were already at their best return loss from when I was working on it the other day. So I went ahead and tuned the notches to a 600KHz split and rotated the loops to get 0.4DB IL per cavity and adjusted the notches back to 600KHz and got 31DB out of the notches. When I called the receive side together, I had almost 7db of insertion loss at the receiver frequency and the notch was way off of where it should have been. Using the notch screws on the 2 cavities closest to the antenna port, I was able to adjust the notch to where it belongs and was getting 90db of notch. However, when I adjusted the 3rd notch screw on the cavity where the receiver connects, all that did was adjust the insertion loss on the pass. I adjusted it to 1.45db of IL and 90db of notch. > > Is that right though? I would have expected the 3rd notch screw to also adjust the notch and not the insertion loss on the pass. > > > > > > -- Untitled Document |
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