FlightAware Discussions

Antenna mounting suggestions

What about this cable, does it pass muster?

https://www.amazon.com/MPD-Digital-Shielded-Cable-Connectors/dp/B07RG7NPLD

If you want original LMR400:
https://www.amazon.com/LMR-400-Times-Microwave-Digital-Connectors/dp/B07P4JCRNG

30 bucks more though.

Not sure about this 3 dB loss figure. My recollection is that it’s around 5% loss, and depending on the total length less, due to the lower attenuation of RG-6 compared to RG8X, the other popular ham coax.

Sorry but you’re wrong, 3dB loss is equal to about 50% of the signal. As a radio ham, this is pretty basic stuff. If you’re transmitting 100W through a length of cable with 3dB loss, you’ll get around 50W out the other end.

As I always say, buy the best you can afford. I see people spend hundreds of pounds (Euros, dollars, whatever) on decent aerials for ham radio but try and cut costs by using the cheapest coax they can find and then blame the aerial. Coax is important.

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I was talking about the transition loss from 75 Ohms to 50 Ohms not that 3 dB is 5% loss. Enough from me!

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Cable Attenuation for 50 ft
LMR400 2.2 dB
RG6 4.9 dB
Difference in attenuation = 4.9 - 2.2 =2.7 dB

So the advantage of using best quality & costly LMR400 over using standard quality & low priced RG6 is 2.7 dB. Adding a masthead amplifier can give you a gain which is 7 times this loss

If you use RG6 50 feet, still the losses can be easily covered by FA antenna gain (> 5 dBi) and receiver front-end pre-amplifier gain. Why to break head for something trival? If you want to cover for 4.9 dB attenuation of RG6, use masthead amplifier, these have gain far in excess of 4.9 dB coax attenuation.

Coax attenution is important in transmitting situstion, where watts, or hundreds of watts are involved, and recovering 3 dB loss requires a trnsmitter with double power output. Recovery of 6 dB attenuation requres increasing transmitter power 4 folds. The cost difference is substantial.

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  1. Each transition from 75 Ω to 50 Ω (or vice versa), causes an attenuation of 0.177 dB. If you have a total of 4 transitions, the total attenuation will be 4 x 0.177 = 0.508 dB

  2. Length of coax does affect performance due to attenuation, irrespective the coax is 50 Ω or 75 Ω. Longer the cable, higher the attenuation. The attenuation per meter length depends on construction of cable (mainly on core insulation material & diameter), and not on its impedance. The RG6 coax has attenuation of about 6.1 dB / 100 ft (20 dB / 100 m).

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see this link re gain versus signal to noise ratio…i apologize if this article (link) has been posted before
https://ava.upuaut.net/?p=836

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" It’s not about the gain its about the signal to noise…"

Indeed, and this is why receiving magnetic loop antennas have such a loyal following.

Using an amplifier up there complicated installation quite a bit.
Also using adapters to F further increases losses.
Terminating RG6 with F connectors yourself also increase potential loss.
If someone is asking for a kit, it’s not really a suitable suggestion in my opinion.

Overall spending an additional 40 dollars for a 25ft cable that fits the antenna and dongle aren’t too much i think.

A good N connector will typcially also be connected rather weatherproof to the cable.
Giving the N connector a secondary wrap with some tape can’t hurt though.

It’s surely possible to negate the RG6 with a masthead amplifier, but it’s quite a bit more complication weatherproofing the rtl-sdr LNA.
With other LNAs, you are looking at more money than for some quality coax.

Fair enough, but he ‘opened’ himself, after the original post, to buying the pieces separate. Any suggestion should be welcomed. With all the info available, he then decides based on his interests, commitments, and means.

That cable assembly you suggested, while excellent, is not easily sourced here, at least at a ‘decent’ - to me - cost.

Part of the enjoyment for me is trying to get things done affordably. I can sure afford some better hardware for ADS-B reception, but it won’t give me any more satisfaction. Are the others wrong? Of course not!!!

It’s only a hobby, for me at least. If you are not having fun, you are not doing it right.

@ALevier15, I know you withdrew the post, but here it goes.

If you are not into tinkering, experimenting, and pardon my Shakespearean english, “half-assing” (in a good sense), go with the best hardware suggested here that you can afford. It’s safer, and less frustrating perhaps.

I’ll be using this waterproof junction box:

That’s an LNA4ALL to show some scale. It’s a bit tight with the RTL-SDR amp in it, but it will fit. Since it’s going to be mounted vertically, I may remove the bottom cable gland which will allow some ventilation and give slightly more space. The amp is high enough inside that there won’t be any water ingress from underneath.

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That has always been my objective, that is why I started with generic dvbt black ($10), RG6 coax from dollar store ($6.99 for 50ft roll), RCA Satellite LNA (13 dB 950~2050 Mhz) for $4 from local electronics store, DIY bias-t using recycled tv splitter box $2, DIY antennas like Cantenna and Quick Spider, $15 Orange Pi PC etc etc.

Don’t forget the eBay ‘junk’. Power inserters, LNAs, antennas, filters, etc.

Didn’t mean to withdraw the post. Was just saying I worked Avionics in the military and dealt with Coax…never knew the topic went this deep.

You are wrong, it is not 5%, it is 4% :wink:

Coupling 75 Ω with 50 Ω results in VSWR of 1.5
See the table below. The reflected power for VSWR 1.5:1 is only 4%

vswr%20vs%20reflected%20power

I stand corrected.:smile:

Most transceivers will handle that with ease. My Icoms will throttle back only at an SWR of 3:1, I think. OK…make that 2:1 to be safer.:sweat_smile:

Also, I read somewhere that the LDG brand of antenna tuners, popular with hams, ‘give up’ once it ‘matches’ to 1.5:1. I have two of those, and it sure sounds plausible. The IC-7300 built-in tuner is much better in this respect.

Bump the gain from 49.6 to max. :rofl:

Sorry, I could not resist.:wink:

P.S. I know, not the same as doing it closer to the antenna.

Just a thought from this morning.

I simply used my currently running indoor antenna and placed it on a can outside of my window and this the result.

number of messages, seen aircrafts and range is going down instead of up as everyone would expect.

Distance between both positions is 1.50 Meters
You can immediately identify the time i did that move (note: range is km, not NM)

image

So i will create a DIY antenna, but let’s see how it performs compared to my quick&dirty solution.
Interestingly the signal level and number of tracks seen did not change