FlightAware Discussions

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

Hi Keith, aye, tis! You have a very impressive setup and your coverage maps speak for themselves. Thanks for taking the time to write that up, it’s been very helpful. Sorry it’s taken me a while to catch up with this topic again. Here’s where I’m up to.

I have the setup you’ve seen running on a windowsill. RPi 3B, ProStick Plus, Uputronics SAW filtered preamp (LNA), then 1m of CLF200 coax to the FA antenna mounted on a high-gain kitchen-roll holder (okay, it was from ASDA).

setup

I’ll be moving the antenna to the chimney. The LNA will be going up there too, and so I’m going to put the 1m coax up there to keep it close to the antenna. That also lets me feed it down and up into the underside of a weatherproof enclosure mounted just below the antenna (whereas a pigtail would be so short as for the LNA to need much more careful weatherproofing considerations I think).

That leaves me with the question of what coax to get from the other side of the LNA down to the ProStick Plus and bias tee. I’ll go with the same bias tee you used, it looks good and comes up on Amazon too. I can grab some GPIO single push pins to make up individual wires, or else hack a USB cable to get just the power, to power the bias tee. Sorted.

So what about the coax. This is confusing because there are so many options. I see CLF200, RG6, LMR400. There are 50 ohm coax runs with a dB/m attenuation. Then there are 75 ohm coax runs with different (lower?) attenuation with the cost of a small impedance mismatch at each end, not a big deal for receiving and especially with the LNA already on the roof maximising signal to noise. I’d prefer to find a source for coax which lets me order a desired length with prefitted SMA male connectors (one end for LNA up on roof, other end for bias tee indoors).

Do you have any general thoughts on 50 ohm vs 75 ohm? RG6 vs CLF200? LMR400? Other types? I don’t need anything special, just as long as it’s technically good quality with the minimal combination of losses (attenuation over length + attenuation from connection including any impedance mismatch losses). The Paoloni cable looks very nice but perhaps that rotor-friendly construction is overkill. I’d prefer to use Amazon UK (or Pi Hut) if what I want is there.

Now, my mate who’s been having the fun with dhcpcd resets… he has the same setup as me and he is also about to put his FA antenna on the roof. In his case his indoor parts are in the attic and so he just needs a 5-6m piece of coax to reach the antenna.

So in his case, although technically he can also move the 1m coax and LNA onto the roof, fit a weatherproof box and install a bias-tee inside, it would be much easier if he simply replaces the 1m CLF200 with a longer piece of coax. Perhaps the same but longer 5m prefitted coax available from Pi Hut.

Here’s a summary of his current setup and the two options.

Anthony from Uputronics very helpfully gave his thoughts on this. He feels that Option 1 isn’t going to present any problem since it’s only 5-6m of coax. The loss of LNA effectiveness from having 5-6m of coax between the antenna and LNA will be fairly low, and perhaps doesn’t justify the hassle of adding in a weatherproof enclosure and bias-tee.

My own feeling mirrors that, but then again since he’s going on the roof anyway to install the FA antenna, if moving the LNA and 1m cable up there too and then getting decent coax down to a bias-tee is able to eke out a bit more from the setup, perhaps it’s silly not to go for it. But how much more can it really eke out by moving to the antenna end of a 5-6m run of coax but with a bias tee now inserting loses into the equation?

It’s hard to make a decision since I have no idea what the realistic differences are between Option 1 and Option 2 in terms of the hassle involved vs the benefit gained from that hassle. The benefit is a combination of signal gains and noise reduction (from having antenna higher up and LNA closer) versus losses from bias tee insertion, or perhaps impedance mismatches and per-metre losses.

If it was 100m of coax then it’s a no-brainer, the LNA has to go next to the antenna. But 5-6m? Worth getting the LNA on the roof to avoid it?

I hope I explained it well enough, thanks and I welcome your thoughts and experience on the matter.

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Good to hear from you again Chris,

The question of 50 ohm or 75 ohm comes up time and time again in here and I can only refer back to my earlier post where I mention that I always try and minimise loss. If you use 75 ohm then you’re introducing a small amount of loss through the impedance mismatch and for that simple reason alone, I won’t touch it.

I noticed last week that Moonraker have introduced a new coax with relatively decent specification. They call it F-Zero and you can see some more details here. They claim 16.71dB/100m (amusingly, they claim that loss figure is 16.71dBd) at 1 GHz. If that appeals to you, it might be worth a call to them to see if they can provide it pre-terminated in the specific length you want. I can’t remember how much you need.

Regarding your friend. If he’s only using 5-6m of coax, I don’t think I’d bother too much about mounting the LNA at the masthead unless it’s easy enough to do.
That’s about the same length I have between my aerial and the receiver in the box and with the M&P cable, I’m getting around 0.7dB loss. If he uses the cable I’ve mentioned above then it’ll be just over 1dB. I don’t know anything about LMR400 so I can’t compare it.

Not sure how helpful that is really!

But I do stand by what I always say - Use the best coax you can afford, even on short runs. When I buy cable, I normally buy more than I need and the offcuts get made into patch leads. I have a whole stack of leads hanging over my door made with different types of really excellent coax!

Keep us updated, I’m keen to know how you get on.

https://www.timesmicrowave.com/calculator

What is your gain now? Adding 5m of coax probably won’t be a significant attenuation, I expect around 1dB, so you could up the gain to the next step and still be more that that. I would try that first.

Is that LNA waterproof? It doesn’t look so, so you will need an enclosure too.

On the flip side, however, I’ve read that 50 ohm has a higher transmission loss for longer distances (I can’t recall where I read that). Therefore, for longer runs on receive-only setups like these, one strategy is to use 75 ohm coax for the lower transmission loss, and swallow a small impedance mismatch at each end, and that still works out a much smaller loss than using even high quality / more expensive 50 ohm coax.

It is quite easy and since there’s a trip up on the roof anyway he’s up for it. We were chatting earlier and came up with a plan. Rather than move the LNA onto the antenna mount and connect it with the existing 1m of CLF200, he could actually attach the LNA directly to the bottom of the FA antenna using a male N-type to male SMA coupler like this one on Amazon. And then cover the entire coupler and Uputronics in self-amalgamating tape and then white electrical tape.

That would waterproof the Uputronics, and since the circuit board inside is already in a spacious aluminium enclosure, it should be fine in terms of heat, with the tape-covered enclosure essentially acting as a small bespoke weatherproof enclosure. The white tape also works best for reflecting sunlight to keep the enclosure heat down (certainly no more than it would be in a separate weatherproof enclosure box).

Then we’ll use this bias tee and connect it to the relevant GPIO pins on the Pi in the attic.

All that means he extracts the maximum performance from his kit and especially from the LNA.

The Moonraker cable looks very nice. I’ll give them a bell later today and see if they can provide a length with pre-fitted SMA male connectors, or else sell the connectors and relevant crimp tool (or solderable ones if that’s doable). Failing that, is run-of-the-mill RG6 cable on Amazon, such as this, suitable for this application (all the listings focus on use for satellite TV)? Or is it garbage?

Agreed, it makes sense since once it’s installed it’s not going anywhere. We are both children of the 70s and 80s and the CB era, so we’re well versed in the fun to be had with decent coax feeding a 36ft antenna versus cheap rubbish. I once had some that had a sheath made from silver foil cuts, and a single strand of copper running along them to electrically connect them, and the centre was a few strands of something pretending to be copper that wouldn’t accept solder in the PL259 plug. Utter rubbish!

Will do, thanks! And I’ll let you know what Moonraker say (is this the same Moonraker that made rigs?) Once my mate is sorted I’ll get mine on the roof too, probably using a bit longer run of coax but again with the LNA right on the antenna.

Be careful how you mount the LNA. I’ve tried this exact setup and the connector on the LNA usually breaks off with any form of wiggling from the cable below. If you have a way to get a shim between the LNA and the pole, you might be able to clamp it down, but the FA antenna and pole mount make it so there’s like a 1/2 to 1 inch gap between the LNA and the pole. I also sealed the whole LNA with silicone and self sealing tape but I still ended up with water in the case. I now use a 1ft section of LMR400 to get into a waterproof box where the LNA lives and then a regular run of LMR400 to get down to my airspy.

I’ve gone through two Uputronics LNAs trying to get this sorted out. I’ve broken the SMA connector off the square brass mount twice (was able to solder it back on) but both eventually died due to water, so now it’s in a waterproof box with the bottom plate taken off and a couple drain holes in the box.

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On my LNA I have created a loop under it and secured the cable after that in a way that cable weight does not pull on the connector.

What I should’ve said more was, If you can find a way to hard mount the LNA to the pole, either with a shim or the lug mount or something, it would last much longer. Any amount of force on the connector seems to make it break off (the threaded portion). I’ve even pulled one out by tightening the nut on the outside of the box slightly too hard.

The waterproof box I have it in now has the LNA secured by the two cables coming in from top and bottom. so the LNA is suspended in the middle of the box with both cables clamped down by the waterproof cable glands. The box is secured to the pole by hose clamps that i cut slits for and then silicone sealed up.
This is in FL, made it through last summer with no issues.

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Safe to say that neither the coax itself nor most of the RF connectors are really designed to be in tension or shear loading. Just plugging stuff into a Raspberry Pi makes me concerned about the creaking of the board and connectors and all the sma connectors look suspiciously delicate.

I was just reading a whole thread on another unrelated board ( lost the link as I wandered off into another internet rabbit-hole…) about coax strung between buildings needing messenger cables and how to tie- and we are doing it all wrong and so on.

And yet I’ve seen unsupported RG58 cable TV coax strung from utility poles to houses all my life, working fine through rain, snow, wind and ice.

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Erk, thanks, I appreciate the heads-up. That’s saved us having the same issues, really appreciated. In that case it’s back to the first plan of using a proper weatherproof enclosure and using the existing 1m of CLF200 coax between the antenna and LNA, feeding the box from the underside. I know nothing about these weatherproof enclosures.

I read your later post about using hose clamps and slits cut in the box. I like the sound of that, seems like a good hack to get the box on the pole (drilling more stuff into the old chimney brickwork isn’t an option).

Do they tend to collect condensation or anything? If there are drain holes, doesn’t that let in insects which can end up getting themselves and moisture into the innards of the LNA? Obviously we’ll still ensure that connectors are taped and the USB connector taped over, etc. I see quite a few on Amazon so we’ll just measure one up, an IP66 rated one should do for longevity.

Is there anything special about LMR400? Just very flexible with low loss at 1GHz? At the moment we have 1m of CLF200 connecting the antenna to the LNA with the N-type and SMA connectors pre-fitted. That’s obviously 3ft vs your 1ft, but I think it keeps the antenna and LNA close enough not to worry about. Then there’ll be 5-6 m more coax after the LNA.

I remember back in the CB days in the 80s, there was a kind of laddish pride in outdoing people with a super smart setup, all technically spot-on and installed really well. And there was a lad down the road with an old antenna tied to the chimney with old rope and dirty old reused coax pushed into a PL259 plug with braid wires all over the place… and his setup out-performed everyone’s for miles around!

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for vents in my waterproof LNA (the LNA is RTL-SDR LNA 1090) enclosure i use these;
VENT-PS1YBK-N8002

they are made by Amphenol LTW and i sourced from Element14

i have been using them for well over a year with no issues

Heh - I resemble that remark! I performed a minor upgrade yesterday and taped the antenna to a different window. If I could reach the chimney I’d probably have taped it there :slight_smile:

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Sounds like my fathers setup from the early 1960’s- I remember his radio buddies searching everywhere for the high-dollar equipment that surely must be hidden behind a door or something- but it was just simple cheap stuff that worked incredibly well.

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I wouldn’t say there’s anything special about LMR400, it gives less loss over distances vs other cables and I want to keep losses to a minimum. It’s fairly flexible but definitely much more stiff than rg6 or other standard cables. I was even looking at lmr600 but that’s super overkill and you might not even be able to get an sma connector on it.

I used this for a waterproof box

I cut 4 slits in it and ran a hose clamp through the top two and the bottom two slots and clamped it down tight to the pole. once it was in place solid, I jammed a bunch of silicone in the slots. (I’m sure there’s a better way to pole mount the box, but this was the quick and dirty I used)
I drilled a hole in the top and bottom and installed waterproof cable glands and pushed the cable through, once everything was secured how i like, I wrapped the cable and cable gland with self fusing tape for extra security.
I drilled some tiny holes in the bottom of the box on either side so if there’s any condensation build up or water ingress, it’ll drain out the bottom.
When I had initially tried sealing just the uputronics with silicone and tape, it filled up with water after about 6 months and literally had water sloshing around in it.

If you were worried, you could drill your holes and glue in a section of screen to keep bugs out. I made my holes small enough that most bugs can’t get in.

£4.37, Free postage

Has anyone tried it?

Yep - I bought two.
One had only a three-leg amp chip (the fourth leg was in the bottom of the package).
The second performed so badly they were both in return post withing the hour.

Also the SMA connectors were the wrong size for the PCB pads

I decided they were a POS and not worth spending time on.


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Correct, you can’t. .

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That’s really not a big deal.
Running a “heavy” cable direct into a piece of equipment can put a lot of strain on the connectors.
The common work-around is to use a short pigtail using flexible cable. This allows you to use the most suitable connector series at each end. In this case, you’d use Type-N on the LMR600 and Type-N + SMA on the pigtail.

Exactly what I do on my receiver. The Hyperflex 10 goes to a very short pigtail into the feeder box.