Will reducing the length of my LMR-400 coax help my range?

When I installed my system I sprung for some Wilson 400 Ultra Low Loss Coax Cable, which I presume is LMR-400. I have a 50 foot, fitted length, need about 40 feet and have a loop of about 10 feet just before the cable enters my home.

I am desperately trying to eek out another 50km from my system. I have identified an alternative spot for my Rpi, though it will be a royal PITA to place there, but doable. If I go this route, I will cut my cable requirement down to about 20 feet, essentially cutting my coax in in half.

The specs on the cable say “Attenuates Signal Less Than .1 Db.” Based on this, and my meager understanding, it does not seem that reducing the cable run will do much for me in regard to signal strength, but I do not know…

Here is my ADS-B signal level from this morning:

So, will I receive any “return” on the effort to shorten the cable? Will it improve my signal noticeably? Thanks for your consideration and time!

You want the attenuation figure at 1ghz, it is usually quoted as dB per 100ft or per 100m, multiply by the cable length.
LMR400 is somewhere around 4.5dB/100ft IIRC. So 0.9dB for 20ft. Not much.

Antenna position will probably have more of an effect on range than 20ft less of good cable so don’t compromise the antenna position for the sake of a slight shorter cable run

Thanks. Working on improving antenna position as well.

Chopping it off is unlikely to make much difference, use the extra length to raise the antenna 30’ - getting it above surrounding trees and buildings will be worthwhile.

Fitting the flightaware filter to my previous stick added 40% to stats and 10% to distance.

At these coax lengths do Power over Ethernet with a remote station, e.g. everything mounted at the antenna.
Or add amplification, filtering, the list is endless, but cheapest and best will be homemade PoE and a Pro Stick Plus with FA antenna, then adjusting gain in software to prevent overload.

50 ft of LMR400 attenuates 2.1 dB at 1090 MHz , and 20 feet 0.85 dB, a 1.3 dB difference


The difference is not big, but every dB counts and 1 dB should give around 10-20 nm increase of range.

But, as other have said, getting the antenna 25 more feet into the air, should make a bigger improvement.


POE can be a very nice solution, but sometimes it’s just not practical to put electronics up in a tower.

Also, “home-made PoE” usually means just extending the DC feed cable, and that is NOT a good idea at 5V 2A.

Active POE is much better, and I prefer af-compliant PoE, but there are cheaper alternatives that also use 48V and a DC/DC converter at the end.


This chart shows that you are losing 1/4 of the power in the coax alone
There are two types of LMR400. LMR400 and LMR400UF (Ultraflex), I use both. UF has a bit more loss but is more flexible.

Antenna height will probably make the most difference.

Are you using the FA filter? A cavity filter generally has a lot less loss and a narrower bandwidth(Blocks local and airborne radar overriding your receiver).
Which dongle are you using? Just curious
I am sure you have already looked into optimising the gain.

When you do your final antenna setup, I suggest that you use an internal seal like this
amazon.com/gp/product/B001U … UTF8&psc=1
and an external seal like this
amazon.com/gp/product/B000Z … UTF8&psc=1
on the N-type coax connector. It will save you a lot of issues in the long run.
These are just brands I picked. If you know of better brands then, by all means, use them.

I also agree that POE to get the antenna closer to the RPI or Odroid (or something similar) is a great choice. I have two in my attic that work really well.

Thanks for the suggestions. I am using the FA antenna and the “new” FA pro stick plus. I am a pretty chunky guy and I have a really, really steep roof. Those don’t mix! I paid about $200 for a jackleg to mount my FA antenna up on my chimney. It is about 6-8 feet above chimney, on some PVC pipe. I am in great shape in regard to structures, but have several trees that still interfere. The biggest block is to the NW and this shows up clearly on my heat maps. I will not be able to get above all the trees, regardless.

I live in a Historical (Hysterical) District, but am known as somewhat of a lunatic, so the Compliance Nazis have not yet visited. I am sure having an antenna, any antenna, sticking up from this old historic house violates lots of rules, but so far so good.

I had scoped out extending my antenna another 12 feet, using Shed 40 PVC in a Schd 80 PVC pipe for stability. Oh yeah, I am in Eastern NC, in Hurricane City.

I have done a lot of gain analysis and optimization … 43.9 remains my friend!

Still, having stated all that, frustration continues so I am going to move out on antenna extension, maybe give the DPD antenna (http://www.dpdproductions.com/page_vhf_air.html#adsbout a try, AND move the RPi upstairs thus shortening the cable run a little. I just have to find someone to install for me. The last guy about fell and is NOT interested. Imagine, I got a guy who will go to the top of a huge pecan tree using ropes and special boots, but he wont touch the roof.

I have searched for comparisons between the FA antenna and the DPD one. Can’t find anything definitive. As ex government employee, my inclination is to “throw money at the problem,” but would really appreciate some words on the DPD antenna.

The connection to my current antenna is not sealed as good as it could be. I have learned a lot since having it mounted. I DID NOT USE dielectric grease and the tape I used is not as good as the example shown. The exterior terminus, at my grounding block is up to the quality of the sealing material shown. It is quite possible that simply “fixing” the seal between the antenna and the cable could fix all my problems, but I have such limited access to get back up to the antenna.
As always, thanks for your time and thoughts!

Hi jcinnb,

Understand your fear of the roof, I won’t go near mine, I’d crash into the neighbor (I’m 230 ish :slight_smile:

Try placing a TV antenna below the FA antenna on the tower, then the “unsighly” nazis might complain less. “I need a TV antenna, you know” :wink:)))

Every less feet of coax cable will improve reception due to less coax loss. However, if getting the best range is in order and you can part with some money, look into LMR600 or better for minimum coax loss.
Or get a PoE raspberry setup mounted at the antenna, cheaper and better, use 18V or 24V charger adapter and 12/24V downconverter (trucker shop) on the other end. Just use a metal box, and keep electronics separate from dongle, throwing everything into one box will increase electrical noise reaching the PSP.
Good luck,


There is no need to build that yourself with the risk of frying your Rpi, when complete kits are available around 20 USD.

amazon.com/TP-LINK-Power-Et … BUB7C?th=1

This one is not AF-compliant, but uses 48VDC.

This splitter is AF-compliant (if you already have a POE switch):

amazon.com/TP-LINK-Power-Et … FATQK?th=1

I have also used these ones, which already have the correct connectors for the RPI:
amazon.com/UCTRONICS-802-3a … +raspberry


Thanks for the link to the amazon one with the micro USB connector! Saves me some work!

be sure what you pick lists 802.3af or at compatibility!

bob k6rtm

You are welcome. It is very handy. If you need a normal 5.5/2.1 mm barrel connector there is a version for that as well.

af/at compatibility is good, but if you buy a kit like my first link it works just as well, as long as you keep them together and don’t mix with other POE equipment.

As long as you select an active solution with 48VDC and a DC/DC it is likely to work anyway.

But af/at has clear advantages, working with most POE switches.

The good thing about af/at standard is, that with a 10/100 switch that puts power on the DATA pairs (instead of the SPARE pairs), and a af-compatible splitter (must accept on EITHER pairs), you can actually split a Cat cable to 2 separate 100 MBps + POE connections, powering 2 devices with separate LAN connections, using only one Cat cable. Verified and working!


  1. I purchased a bunch of PoE stuff early on, using recommended components from this and other ADS-B fora. Just could not figure some stuff out.

Can anyone recommend a GREAT tutorial on this.

Secondly, how does PoE work in relation to grounding? Currently with the LMR 200 I have a grounding block. Is the Ethernet cable grounded?