Radar Problem

[keithpitcher]

We have an Apelco LDR9900 radar that was fitted to the boat when we bought it last year, but have yet to get any meaningful targets to display. On the lowest range as the gain is increased dots appear close to the centre of the display in a ring. As the range is increased these dots move closer in as you would expect. As the gain is increased dots appear over the whole screen until the screen is pretty much completely covered but the whole pattern appears to be random, This has been replicated in marinas and open water - and in range of suitably large vessels that should provide a clear target. The sweep appears to be working and sea clutter and rain clutter functions have an affect on the display so I assume they are doing what they should.
Can anyone suggest what I could check on the radar or what might be causing the problem - or suggest someone in the Gosport area who might be able to help?
Thanks.

 

[tt65]

Not, sure but sounds like tuning. Does set have any form of self testing that may diagnose for you?

 

[[27631]]

Hi....could well be the magnetron in the scanner is knackered!!Probably £400 + if it is!!

 

[radarman]

Hi,

It does sound like you are not actually putting out any real RF power. The returns you are seeing on the screen sound like the effect of false alarms caused by (normal) noise in the Rx chain.

A really good test would be to get hold of an RF monitor (NARDA probe) and check to see if you are really transmitting any RF.

 

[tt65]

Hi again, saw somewhere that this set has manual tuning. The tuning MUST be correct to receive anything as a return echo. Will still receive noise though.
Tim

 

[keithpitcher]

Hi Tim,
Thanks for the reply - are you referring to tuning the local oscillator?

 

[tt65]

Yes, the local oscilator should be tuned accurately. Difference in frequency between the magnetron and the local oscillator produces an intermediate frequency that goes through filtering and is the signal that is processed to produce targets on the screen. If it is off tune the filtering will reject the resulting wrong frequency.

Should be some indicator for tuning but in my days in radar servicing some sets allowed tuning while viewing screen to maximise the echos. (Long long ago)

We also put a small neon lamp in front of scanner which would light up if magnetron was transmitting, do not try that!
We also put hand across waveuide end, if hand warmed up the magnetron was working? do not try that!
We also looked down waveguide to see if spark gap was functioning! Do not try that.
Actually those tests probably not possible at all on small radar.

Having thought about it I might have a go with my own sometime to see if a neon taped on radome will ignite, that could be fun! but might not be close enough.

Sorry about the remeniscing.