Swanwick Military 277.775Mhz

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timbo
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Swanwick Military 277.775Mhz

Post by timbo » 14 Jan 2015, 11:24

Been picking up the ground controller for a while now, very bottom of the box - no signal ,only weak audio, with an Airmaster (nest of dipoles type) Civil/Mil antenna. I've now installed a 5 element Yagi for Mil Airband. Thing is , which way do I point it ? According to the interweb 277.775 is coming form Great Dun Fell and High Buston, seems strange to have two transmitting the same frequency in a similar part of the country, would have thought it caused doubling at the aircraft end. I'm only getting the airside on 135.075, no controller.

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Beefi
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Re: Swanwick Military 277.775Mhz

Post by Beefi » 14 Jan 2015, 14:50

Point your Yagi at one site and measure the signal strength, and then point it at the other and do the same. Then fix you Yagi to the site that gives you best performance.
It would not be unusual to have multiple sites on the same frequency. When a signal is received it usually votes on the best site signal strength and then responds with that sites transmitter. Their antenna layout or direction could also be different. Also using AM reception is a little more tolerant to two transmitters at once unlike FM which would have a phase cancelation effect. I think thats why aviation stick to AM, it is possible to have two transmissions at the same time on the same frequency and be able to get usable information. You could always buy a rotator then you can have the best of everything!

And where did you find a MIL UHF Yagi for sale?

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thedeerhunter270
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Re: Swanwick Military 277.775Mhz

Post by thedeerhunter270 » 14 Jan 2015, 15:39

On VHF airband they do transmit what seems to be the same frequency from different sites, but I think they vary the frequency by a few k at each transmission site. So, one of the UHF sites may be on 277.770 and one may be on 277.775.

Edit:
http://www.air.radiouk.com/page39.html

Yep, Great Dun Fell on 277.772.
Operating in South Northumberland and Weardale.

Militaryspotter7
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Re: Swanwick Military 277.775Mhz

Post by Militaryspotter7 » 14 Jan 2015, 20:26

277.7750 is Scottish mil icf east. so basically from Lincoln right up to northern Newcastle.
has nothing to do with great dunfell etc its widely heard as its a primary frequency

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timbo
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Re: Swanwick Military 277.775Mhz

Post by timbo » 14 Jan 2015, 22:01

thedeerhunter270 wrote:On VHF airband they do transmit what seems to be the same frequency from different sites, but I think they vary the frequency by a few k at each transmission site. So, one of the UHF sites may be on 277.770 and one may be on 277.775.
Yep, Great Dun Fell on 277.772.
Yep, That makes perfect sense, I know that they sometimes offset by a couple of khz.
Militaryspotter7 wrote: has nothing to do with great dunfell etc its widely heard as its a primary frequency
Has quite a bit to do with Great Dun Fell, as this is where the transmitter site is located. What used to be Scottish Mil, is now Swanwick Mil on this frequency.
Beefi wrote:Point your Yagi at one site and measure the signal strength, and then point it at the other and do the same.

And where did you find a MIL UHF Yagi for sale?
Seem to get better results pointing North, so this would indicate the High Buston site.
As for the Mil Yagi --- Retuned DAB antenna (220mhz) ......with some Yagi design software, tape measure, hacksaw and a drill ...Hey Presto ! Cheap Mil Yagi.

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Minus1
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Re: Swanwick Military 277.775Mhz

Post by Minus1 » 15 Jan 2015, 09:08

When multiple transmitters use the same frequency, each is offset by a few kHz to avoid/reduce interference.
If two transmitters are used the offsets are -5kHz and -5kHz
If three transmitters are used the offsets are -7.5kHz, 0, + 7.5kHz

Most receivers don't have any way to fine tune to these frequencies, but you can often figure out which is relevant by inputting the five possible frequencies into 5 successive memory channels and seeing which is best for you. Although most receivers can't distinguish between a 5k offset and a 7.5k one.

For example at Clee Hill the offsets I know are:
232.875 -5k
252.875 +5/7.5k (reciever unable to tell which)
278.6 +5k
280.35 +5k
292.525 0k (there may only be one transmitter)
372.625 -5k
133.9 +5k

This explains why it is common to hear aircraft having difficulty receiving the controller when you can hear both clearly.
You'd think that these days NATS could have the transmitters automatically or manually switch their offsets depending on where the aircraft they are calling is.
Or they could define a dedicated ICF for EACH transmitter site, and have the aircraft automatically use the nearest site's ICF — but apparently such technology is beyond the capabilities of a multi-million pound military aircraft.
KEY : = channel/stud | ~ = CTCSS/DCS | ^ = transmitter site | ¯ = overhead | * = trunked

pete_uk
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Re: Swanwick Military 277.775Mhz

Post by pete_uk » 21 Jan 2015, 16:48

um...

Why do they have offsets in the first place?

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thedeerhunter270
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Re: Swanwick Military 277.775Mhz

Post by thedeerhunter270 » 21 Jan 2015, 17:02

pete_uk wrote:um...

Why do they have offsets in the first place?
So each transmission doesn't interfere with each other. I think two transmissions on exactly the same frequency may zero beat with each other and cause an extra tone. Something like that.
Operating in South Northumberland and Weardale.

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Re: Swanwick Military 277.775Mhz

Post by Skirge » 28 Jun 2016, 14:06

I am new to this and have just been receiving a very strong signal on 277.775, he was close (Newcastle) but he was getting a reply on a much weaker signal on 277.783

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