Need help with double discone

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Need help with double discone

Postby ben2311 » Fri Jun 15, 2018 10:25 am

hi, have been given a double discone to get me started, so i can listen to civil/military airbands and ads-b. I know it wont be as good as a dedicated antenna, so what splitter do i need and and is there any way that i can clean the signal up a bit on those frequencies? Will be running it on a westflex 103 to cut down on loss. Any advice appreciated as i'm a complete newbie :sad:
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Re: Need help with double discone

Postby m0lsx » Fri Jun 15, 2018 11:37 am

A splitter as the name suggests, splits a signal. Thus reducing each signal & by more than just the 50%-50% loss of a two way split. As everything has what is called an insertion loss & that includes extra connectors. So add (insert,) anything into your coax run & you will loose a % of the signal. This may be an acceptable trade off for you. But only you can decide.
Buying a good quality well made splitter will make a difference to what you loose.
A good band specific amplifier at the antenna end of the coax, will improve things. But the issue here is that you do not want to amplify a degraded signal, when a big percentage of the signal has already been lost by the split & thus the noise to signal ratio has increased.
So a splitter & two pre amps are not ideal. And be aware. All that a so called broadband amplifier does, is make everything louder. So noise, signals many MHZ away from what you want to listen to, everything. These broadband pre amps, can all too often add in issues that are best avoided.
And avoid anything that claims to be a broadband pre amplifier &which uses the word professional to describe it. As no competent professional would use something like that. Other than as an acceptable bodge & even then. Only after some careful thought.
Another thing you can do to improve a signal quality, is use a band pass filter at each radio, this will clean up what your RX see's. But on Mil Air that will be less than easy to do & probably very expensive to do well.
There is always a trade off in splitting a coax feed, so what you have to decide is, is it worth the effort for you?
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Re: Need help with double discone

Postby ben2311 » Fri Jun 15, 2018 12:38 pm

so would it be better to just run the discone for the civil/military and get a stand alone ads-b antenna? I have also got a half wave dipole antenna would this be better then running the discone? Do i need to run westflex 103 to both or can i get away with just a rg58 for the civil antenna?
This is the dipole that i've got.
https://www.ebay.co.uk/itm/Airband-Scan ... SwQb1a92ns
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Re: Need help with double discone

Postby m0lsx » Fri Jun 15, 2018 1:51 pm

A stand alone ADS system, with a cheap bandpass would give you much better results yes.
RG-58 is a compromise coax on VHF. But if you are only running a very short run of RG-58, 10 meters or less, on Civil Air band, you will not notice much difference. But by the time you get to 20 meters the difference between coaxes, start to build quickly.
I think for Civil Air band only I would be tempted by good quality double shielded RG-8. It is small, cheapish & the loses will be small provided you are not running a very long length.
When you say you have a half wave dipole, what is it half wave of?
If it's civil air band & if you know it's a resonant air band half wave. Then yes it should be good on that band. But some of the RX only antennas that sold as being resonant, are not.
To give you some idea of coax performance.
Good quality (Belden) RG-58. So not the rubbish available from many CB retailers. looses 25% of it's signal over 10 meters at 130MHz. Which is not terrible. W103 will loose around 10%. So the difference at the needle is minimal between the coaxes at 10 meters.
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Re: Need help with double discone

Postby m0lsx » Fri Jun 15, 2018 1:55 pm

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Re: Need help with double discone

Postby ben2311 » Fri Jun 15, 2018 2:43 pm

ok, thanks for the help, will run the dipole and get a collinear for the 1090mhz. So the westflex would be overkill for the dipole as i will only need around 10m, well that will keep the other half happy then as i won't have to spend as much.
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Re: Need help with double discone

Postby m0lsx » Fri Jun 15, 2018 3:16 pm

If you only want 10 meters for airband, then I would go for for RG-8 mini. It is not "Super" low loss as some CB suppliers claim. But it is as good & almost as good as RG-213. Which means it performs very well at VHF.
10 meters of RG-8 mini should cost you about about £7, plus postage. Or 70p per meter is what Westlake, a supplier of quality Coax is charging.
And make sure you get the right size connectors for mini 8 too.
Westlake are charging 50p per meter for RG-58.
So for an extra £2, over RG-58, you get 10 meters of mini 8, which has close to RG-213 performance & save yourself £1.30 per meter. As Westflex is about £2 per meter.
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Re: Need help with double discone

Postby ben2311 » Fri Jun 15, 2018 4:23 pm

will get 10m of rg8 mini 8 ordered and 10m of westflex 103
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Re: Need help with double discone

Postby G4RMT » Fri Jun 15, 2018 5:11 pm

Westflex on the double discone would be my vote for listening to the aircraft - Its won't be much use for the 1GHz+ microwave bands at all - so, if strong ADS-B signals are a requirement, then for me it would be a separate antenna and feeder. As Alan said, splitting is not trouble free, and splitting an already compromised signal doubly bad. I have a receiver for ADS somewhere, but for the life of me I cannot remember where the antenna ended up - I think from memory, it was a short, antenna in a tube sourced from ebay.
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Re: Need help with double discone

Postby Chris P » Fri Jun 15, 2018 7:45 pm

As usual G4RMT and m0lsx are giving some very sound advice so I concur with their comments but add that my Cross Country Wireless Multi-coupler works very well for me feeding several receivers from one antenna with very little loss and good separation between the 5 outputs however the cost of about £118 may be a bit over the top and a separate ADSB antenna ( a home made collinear version) would be my choice .


PS I have a spare redundant AIRNAV Antenna which you could have for the cost of postage
Regards Chris aka G8FFF nipper or tazmin88
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