Another What's This?

Anyone know what this is? Seen on the airfield at Cambridge, seemingly now known as Cambridge City Airport as opposed to Marshall's Cambridge Airport.

Bill,
It looks like an Inner Marker beacon for the ILS, but....

a) The IM is normally just outside the airfield - the pic suggests it is too close to the airfield.
b) Why is the antenna at some strange angle. Two possibilities:

• It's fallen over
• It has been configured for mixed polarity.

With the old wartime SBA system the Main Beacon had vertical aerial elements, and the aircraft had a vertical aerial to receive this. The SBA Inner and Outer markers had horizontal elements, and the aircraft was equipped with a horizontal rod antenna - often described as a 'handrail'.

Then along came VHF BA for using in single-seat fighters, using only the aircraft's R/T set. The aircraft R/T aerial was a vertical element and was therefore insensitive to horizontally-polarised marker beacons. They didn't want to add an extra horizontal aerial to the fighter, so the solution was to move the ground marker beacons well to the side of the runway centre-line axis, and angle their antennas accordingly. An angle of 45 degrees would be optimal. A far from ideal solution but necessary at the time.

Having said all that Option 1 above seems likely.

Where is the device physically located?

Graham

(I do like these 'What is this?' threads)

Graham, it is well within the airfield perimeter - you can just see the runway in the background. The control tower is some 150 yards directly behind the photographer. The shot was taken with a telephoto lens and the building was about 50 yards away. My immediate impression was also that it had fallen over!

Bill

Does this help any? if its still used it will be on here

www.nats-uk.ead-it.com/aip/current/ad/EG...AD_2_EGSC_2-1_en.pdf

www.nats-uk.ead-it.com/public/index.php%...id=39&Itemid=88.html
Stevo

Stevo: Great plan but it is not shown. The building is roughly half-way on a line drawn between the radar head and the tower. This does seem to support the view that the aerial may be disused and has fallen over!

Interesting and useful chart Stevo (do you have one for Middleton-St_George?).

The Cambridge one shows the ILS localizer just off the end of Rwy05 - frequency 111.3 Mhz.
The Inner Marker should be 1,000 feet off the other end, i.e. Rwy23 - certainly not just in front of the CT.
The plan may not show the IM as they were 'standard' items operating on 75MHz.
The length of the elements in Bill's photo, (2 metre-ish) however suggests something around that frequency.

Hmm :confused:

Does Essex Girl, or EGDGZTCW have a contact there?

Graham

Not an old (and damaged!) RDDF is it? The building looks right for it, even if the antenna seems a little tired!

Inner markers, in civilian use anyway, died out decades ago, as did MMs and most OMs, to be replaced by DME. This could be what is known as a Far Field Monitor. In other words it monitors the ILS signals and if they are compromised by a vehicle etc breaking the beam, it activates an alarm on the controller's desk and can "trip out" the ILS if the disruption is more than momentary. This avoids bends in the beam at critical stages in an appproach. The controller can reset the ILS but it can take 30 secs or more to radiate again. The aerial at Liverpool, however. looks like an ordinary TV aerial!

Carnaby;31140 wrote:

Interesting and useful chart Stevo (do you have one for Middleton-St_George?).

Hi,

You can find charts for all current active civil airfields on the NATS website;

www.nats-uk.ead-it.com/public/index.php%...&id=6&Itemid=13.html

There is one for Durham Tees Valley Airport.

Stevo

Dave wrote:

....it monitors the ILS signals and if they are compromised by a vehicle etc breaking the beam, it activates an alarm on the controller's desk and can "trip out" the ILS if the disruption is more than momentary. This avoids bends in the beam at critical stages in an appproach. The controller can reset the ILS but it can take 30 secs or more to radiate again.

I'm forever breaking ILS beams, its the only fun I get at work!

However, normally this building is located adjoining the localiser arials which are at the end of one of the runways.

I'm pretty convinced its an old RDF building (I would put my partners life on it, so 50/50 at least), size and design are very similar, location would also fit quite well.
These were replaced by DRDF a while back, and now there is a newer system coming on stream.