FlightAware Discussions

Building a new receiver, this is going in a prime location - Suggestions for receiver/preamp/filter please

I’m fighting to stay at or ahead of the other local feeders. Every new connector is something like -0.5db, correct? I could have a more flexible setup with pigtails, but I want to minimize loss as much as possible.

The run isn’t long enough for me that going to 600 with a pigtail on each end would be more beneficial I believe. I only have maybe 25ft of LMR400 to get to my airspy.

The actual loss will depend on the quality of the connector and the skill of the assembler - If you haven’t measured it, you can’t really know.

Using the Timesmicrowave calculator, over 25ft, you are looking at 0.5dB difference between LMR400/600
Obviously if you use 50c chinese connectors, the benefit of expensive coax is lost.

I use MPD Digital to make any cables for this. They seem like great quality cables.

That calculator is quite nice though. I’ve been messing with it to see if I’d like to try different configs.

Hi @Dav5049915, thanks for your feedback on the coax. My mate opted for LMR400 and bought all the bits. But it turns out that it has a very thick inner core and it’s very stiff to bend. It’s way thicker than we expected. This means the crimpable SMA male connectors he bought won’t fit. The crimpable N type male connector looks like it just about fits.

There’s going to be about 0.5m (or even shorter) of coax between the antenna and the enclosed LNA, and then about 5-6m of coax from there to the bias tee indoors. Here is the original schematic showing the LMR400.

My feeling is that given that a) we have a very short run from the antenna to the LNA, and b) the longer run is after the LNA and is itself is only 5-6m, I don’t think the coax is huge deal. But since we have this opportunity to do it right and minimise losses, that’s the plan.

He now has two realistic choices:

  • Buy thinner coax like RG8 or RG6 or CLF200 which the SMA connectors will fit onto.

RG8 keeps everything at 50 ohms and transmission losses are negligible given a+b above. RG6 at 75 ohms is not a problem, but doesn’t seem to gain anything in terms of transmission losses over such a small run, and also introduces a small impedance mismatch at each end. Nothing to write home about but looks easy to avoid by simply going RG8. CLF200 is classed as low loss and intended for GHz signals and is a bit stiffer but is also 50 ohms and supports the SMA connectors. It never seems to get mentioned much in these forums. Is it not so common in the US?

Processing all that, my gut feeling is RG8 will be fine. I welcome people’s thoughts on that given the above.

  • Stick to the LMR400 and use N type male connectors on all 4 ends. Buy 3 N type female to SMA male adapters to effectively turns them into SMA male connectors. Right-angle adapters are available.

This has the advantage of keeping the LMR400’s low loss, but introduces new losses in the form of these SMA adapters. The physical flex of this cable looks like it may put a lot of strain on the SMA connectors.

My gut feeling on this option is that the cable run is so short, and given a+b above, the low transmission loss of LMR400 doesn’t offer much here (we’re not talking hundreds of feet where it would shine) and the adapter losses may well more than offset them anyway, plus the physical requirements and potentially larger enclosure are a hassle.

By the way I appreciate that in practice these options aren’t going to matter that much. The main advantages here will come from getting the antenna up high and the LNA close to it. I bet in practice any detriment from coax and connectors is minimal. But given that we still have all options open to us for the coax and connectors before commiting it to the roof, choosing the parts which minimise losses makes the most sense.

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Since you already have the LMR-400, a better option - buy the SMA connectors meant for LMR-400.

Here is one example:

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Thanks @mikkyo, I didn’t realise they existed. I searched before I wrote the above but was looking for the wrong thing. The link you gave is in the US but I’ve now found the same in the UK. I agree it makes sense to stick with the LMR400 if the connectors are available without needing to mess with adapters, and if the physical flexing of the cable is manageable.

https://www.rfshop.co.uk/sma-plug-for-lmr400-crimp

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