How to reduce the signal attenuation caused by long cable?


I am on the third floor of a building (a total of six floors), the antenna is placed on the southern balcony. Currently I use a 5m (16.4 feet) coaxial cable to connect it to the rpi.

Since I want to expand the coverage of the north, I want to place the antenna on the roof. The roof of this building is difficult to access, so for future maintenance reasons I can only place the antenna alone on it (not with the rpi). This takes about 20m (65.6 feet) of cable to complete.

I am very worried that this will lead to a serious signal attenuation, but I do not know how much it will have an impact, but also how to minimize attenuation.


20m should not be a big factor if using good quality cable. I am currently using about 30m of LMR-400 and getting satisfactory signals using the Flightaware 1090 antenna, 1090 filter and standard (cheap) sdr. The LMR-400 is around $1/foot maybe less.
Many that have ADS-B receiving stations are using RG6 feedline commonly found in TVRO satellite type or cable TV cable. This cable is substantially cheaper and readily obtained. I don’t have experience with it regarding 1090MHz reception, I am sure someone on the forum would be glad to share their opinion. Maybe there is a supplier of N connectors for RG6 for your antenna connection too… As in all transmission line assemblies, keep the number of connections as minimum as possible. Each connector/adapter adds more losses.


My recommendation as well would be to go with LMR-400 (and if possible, use a dongle with an SMA connector rather than an MCX; I’ve not had too much luck finding MCX connectors for LMR-400 but you CAN find SMA male easily enough; PAL connectors are even worse, and not optimised for UHF much less the frequency ranges in ADS-B tracking).

My present setup is FlightAware 1090 card, roughly a foot of LMR400, and recently swapped out the old (and starting to die) dongle with one of the Flightaware Pro Plus (mostly because there is a filter built in). Pretty much just the change to the new antenna and LMR-400 (and an N to MCX adapter) nearly tripled planes seen–enough that the (dying) dongle was actually starting to lock up :smiley: (Oh darn, excuse to get something a bit newer with far easier to find connectors!) With the Pro+ dongle actual sightings do seem to have quintupled from baseline.

(The original setup, in case people are curious, was with a homebrew “spider” groundplane, which had N connectors, and original cable was RG316 (higher grade version of RG174, and unfortunately about the highest diameter of cable I could find with native MCX connectors :P).

About the only things I could do to really optimize things further in my location is to go from LMR-400 to Heliax hardline (which is considerably more expensive, and finding native SMA connectors starts getting problematic) or to go for longer cable and set the works outside (which is going to require negotiation from the SO, which is a little more problematic than getting a foot of Heliax and appropriate connectors and probably more expensive as well :D)

Another thing to watch specifically about N connectors–N connectors for RG-6 take 75ohm N connectors, not 50ohm (RG-58/LMR-195, RG-8x/LMR-240, and RG-8/LMR-400 take 50ohm N). The two types of N connectors are actually not compatible (the 75ohm N has a smaller inner connector bit on the inside, and as a result a 75ohm plug won’t make good electrical contact with a 50ohm jack) and the FlightAware 1090 antenna IS designed with the 50ohm N connectors so that’s another reason to just go with the LMR-400. (If I were doing setup of scanner antennas where I’m either building my own or building out stuff which tends to use BNC connectors, that’d be a different thing; the 50 and 75 ohm versions of BNC connectors are at least close enough in tolerances that they’ll work with each other, and I DO keep some RG-6 quad-shield and compression BNC and F connectors and installation tools!)


@ve4gls @windigofer THX!

I’m using N - SMA, Fifty ohms, 5mm diameter cable now.

I will go to see the LMR400.



Size Frequency Attenuation/100m Attenuation for 20m Connector at Antenna Connector at ProStick
RG6 1000Mhz 22.4 dB 4.48 dB N-male to F-female on antenna + F-male on coax F-male on coax + F-female to SMA-male pigtail on dongle
LMR240 1100 Mhz 27.5 dB 5.5 dB N-male SMA male
LMR400 1100 Mhz 14.8 dB 2.84 dB N-male SMA-male


Thank you @abcd567 for the table, was looking for one but to lazy to make one up. :),
Looking at specs for RG6, seems logical to avoid it for long runs even though really good RG6 would likely have 2-3dB less attenuation.
Also with using LMR-400, I would recommend having a dealer or radio shop install the connectors unless you have the proper equipment and experience. You will need a male N connector for the antenna connection outdoors, and make sure some weather proofing is applied to that connector. IMHO best to use the same connector at the other end too and a use a pigtail to adapt to the dongle to minimize stress on the small MCX/SMA connectors.


stress? Physical pressure (tension) or electrical signal pressure?

Use pigtails seem will affect the signal…


Physical stress on the dongle and RPI connectors is the issue. Short pigtail maybe 10-20cm long and the extra connector will only have slight increase in signal attenuation, Better compromise than breaking the MCX/SMA connector on the dongle, or damaging the USB connector on the Raspberry Pi.


In fact, I have a rather special dongle, like this:
(Please ignore the red arrow and the red box below)

In fact, I’m not very clear about its performance indicators … :sweat_smile: But I will consider adding a pigtail, thanks~


Nice set-up! Like the enclosure. Looks like an SMA chassis connector where your antenna lead attaches. Same issue though, the small SMA female chassis connector could easily be broken off with the large feedline connected directly.

For a neat and robust connection, one could replace the existing pigtail between the Feeyo board and chassis with one that has a female N chassis connector instead of the female SMA connector currently in place. It would have to be longer, not sure it would fit where the chassis connector is now, but maybe would fit close to the area highlighted (red reflection from the LED), and would require drilling a hole to fit the N connector. This requires some work and disassembly and re-assembly of the whole enclosure… just to eliminate one connection.


Great idea! Only I was unable to modify the enclosure due not have the proper drilling equipment. In addition there are some contractual restrictions that I can not carry out such destructive operations. :frowning: So I think I can just add a pigtail adapter.

Feeyo is is a Chinese aviation data company. Like FA, they are recently expanding their ADS-B coverage network. I currently use the antenna and receiver they provide to provide data to multiple upstreams, including the FA. (FA rejected to give a receiver to me, :joy: sad)



My Setup showing antennas, RG6 coax & pigtails



I do appreciate that you did show the table (which is why I do tend to use RG-6 in situations where I can use a 75ohm plug that will work with a 50ohm jack).

My main concerns with the setup as illustrated would be that there’s some areas that do introduce additional loss that might cancel out the RG-6 advantages:

a) My biggest would be with the N-to-F adapter on the antenna. If that is not specifically a 50ohm N to 75ohm F, you have pretty much the same issue I mentioned earlier–a 75 ohm N plug has a thinner inner conductor:


The tolerances ARE different enough that inserting a 50ohm N plug to a 75ohm N jack is capable of damaging the jack–and a 75ohm N plug in a 50ohm N jack is not going to make good mechanical or electrical contact (and thus is not going to work as well as with a 50ohm N plug :D).

In my experience, N-to-F adapters tend to be 75ohm all around, and I do know the FlightAware antenna is actually using a 50ohm N connector.

b) The F-to-SMA adapter also introduces some losses (not just from the adapter itself but the shift in impedances)–that’s not nearly as severe as the issue with the potential N connector mismatch, though.

The Cantenna of course doesn’t have the potential mismatch issue (as it uses a native F connector) and if the FlightAware antenna used some other type of connector at the base (BNC, SMA, pretty much anything really but N or TNC) I’d not really sweat the impedance mismatch for listening as that’s just a decibel or two. The worry I’d have is that the better performance of the RG-6 would be nullified by the fact the inner conductor of that N adapter’s just barely making contact at all!

TNC, this isn’t an issue (because 50ohm and 75ohm TNC adapters just aren’t cross-compatible at all!) but you DO have to watch this with N, and because of the fact there IS enough of a difference between 75 and 50ohm N connectors that one risks breaking something or having a bad connection by forcing things…I do try to match impedance with N adapters in particular.

Now, were I doing a similar setup, I’d probably be running LMR-400 if I reasonably could as it’s less attenuation overall, but if I couldn’t…probably a saner approach (that wouldn’t risk a dodgy connection with the N connector) is to get a specifically 50ohm N male to BNC female adapter, and then use a 75ohm BNC connector on THAT end of the RG-6 cable (yes, you can get compression-fit BNC adapters that work like compression F connectors, my usual go-to when dealing with scanner antennas is RG-6 quad-shield with BNC on both ends or BNC on one end and F on the other). The tolerance issues (and risk of breaking things) are nowhere near as severe with BNC as with N connectors.

To the original poster: I don’t know how close you are to some of the markets in Shenzhen/Guandong or Shanghai, but you could probably pick up a right angle N female to SMA male adapter (there’s a little loss but not quite as much as with an RG-316 pigtail; as your receiver is in a dedicated box with a bulkhead connector to outside I’m a little less worried about strain relief). Also you could probably have an N to SMA connector cable made up, I know there are plenty of places on both Amazon and Ebay here in the States that sell them and on Ebay in particular a lot of the sellers are Chinese based. (Alibaba and Baidu almost certainly have sellers if you’re not near the major electronics market areas.)

The cables might not be sold as LMR-400 or RG-8, by the way. Really high grade is RG-213, a common brand here in the States is Belden 9913, and you’ll probably see stuff like CA-400 or KRG-400 or the like. Same really goes for RG-6.



This is the N-male to F-female adaptor I purchase to connect FA Antenna to RG6 coax.
It is working perfectly well since 2016.

Traffic Increase

Here’s my current installation:

First image, FlightAware ADS-B antenna is just to the right of the anemometer. Below it is a 50MHz loop, behind the anemometer is our 5.8GHz ISP parabolic. The ADS-B antenna is at about 48’ above ground level.

Second image: RPI3B and R820T2 dongle in cabinet behind router. I removed the cover from the dongle because it was losing sensitivity after 2-3 hours, was thinking it might be getting too hot. One of these days I’ll take it out and replace the cover. Note the right angle MCX connector/pigtail running to the left and down. This is the same pigtail at the top of the last image in the nest of network cables. Note that between the male N connector from the LMR-400 and the filter I have two adapters. I do have a cable coming in to replace the two adapters to eliminate one connection.
Not having the cover on the receiver dongle gives a good impression of how delicate the little circuit board is…
edit: I always assume the use of 50 ohm N connectors… One would not want to mix 75 ohm with 75 ohm as they are really not compatable, you risk breaking the center (female) or not having a connection at all!


You may want to consider putting the dongle and RPI in metal cases.
I did it to reduce the RF interference.
I like the fan. I use one on my RPI in the attic. It would get too hot in the summer and throttle back.

You have great range. My only suggestion would be a cavity filter(Much narrow filter and less loss than the FA filter) and/or amp/FA dongle with built-in amp.
I use a hab/nevis amp with a cavity filter. My range is limited as my antenna is only 30m AMSL and I get splattered by NYC tourist helos, 500W birds flying over at 1500-200ft(I am on the flight path to La Guardia and NYPD and S76s flying past at 150-200ft.


I put the Wilson connector on my Amazon wishlist. There is no Wilson model # but hopefully that is it. Soon will buy the FA antenna with it.

Recently bought a 400+ ft roll of RG-6 for $10 at a thrift shop, so i am using that into the foreseeable future. Short runs, less than 50 feet.

I also use RG-6 for amateur radio HF, a big no-no i’m sure. Indoor & attic mag-loops mostly < 5 watts WSPR beacons. The 75-ohm cable introduces impedance problems, but i tune the loops for minimum SWR, and if necessary trim the length of the RG-6 to get a match. Budget operations.


For immobilizing cables so they don’t damage fragile connectors: i use “cable tie bases” and cable ties, stuck to the shelving. If the bases come loose, i use superglue to re-affix them. Not ideal for keeping furniture pristine, but it works fine for me.


Wilson Electronics adapter 971128

For tranmitting, impedance matching is very desireable. An impedance missmatch means that a 100W transmitter may be able to transmitt say only 70W. To transmit 100W with impedance mismatch, one may have to use a 150W transmitter at substantial higher cost.

In case of receiving, since the power involved is in micro Watts or milli Watts, adding a low cost rf amplifier such as satellite in line amplifier, or Pro Stick’s built-in rf amplifier, can cover for mismatch losses at negligible cost.

Calculation of signal loss in dB for a 75 ohm equipment connected to a 50 ohm equipment:

The FA antenna has an input impedance of 50 ohms, and RG6 coax has impedance of 75 ohms, a mismatch occurs.

However, the mismatch loss when using 75 Ohm cabling with 50 ohm antenna will be very minimal at around 0.177 dB. So dont worry and mix the 75 ohm with 50 ohm system.

How I arrived at figure of 0.177 dB?
When 50 ohm mixed with 75 ohm,
Reflection Coefficient Γ = (75-50)/(75+50) = 0.2
SWR = (1+Γ)/(1-Γ) = (1+0.2)/(1-0.2) = 1.2 / 0.8 = 1.5

Mismatch Loss in dB = -10 log (1 - Γ²) = -10 log (1 - 0.2²) = -10 log 0.96 = 0.177 dB

Just got a 5bi antenna

Being cheaper than Wilson Electronic one, I have also purchased N-male to F-female adapter from China through eBay, and these also work OK.
These are cheap, but Chinese sent these to Canada by a sail boat :), so it took about two month to reach me.