Why the HF feeds are sound to bad ?

[jeanlottie]

Hello everybody, i'm newbie in this pages, and my question is why the HF feed are listen too bad.
if you listen for example the KLAX Feed or KJK Feed i hear good (5/5).i think is maybe because the Receiver location is to long from the station or something or the person have the receiver no are in the right frequency.
well i hope somebody can explain me. Thanks

[AeroBill]

Jeanlottie,

   Here are a few reasons: 
   As you mentioned the feeds for the VHF (very high frequency) AM aircraft band are normally close to the source
   transmitter as far as ground stations are concerned. This is particularly true of those feeds from Airport control
   towers. Reception of aircraft can be up to 100 miles mainly depending on height of the aircraft and location of the
   receiving station of the feed provider. Also, the feed providers have the squelch control turned up to eliminate
   background noise (hiss/static) so they can scan more than one frequency. Same if they have the scanner
   parked on just one frequency. Normally the ground/air signals are strong enough to "break squelch" and easily be
   heard on the scanner without having to constantly listen to background noise.

   HF (High Frequency) feeds must pick up very weak signals from distance aircraft, or ground stations hundreds of
   miles distant from the receiver site. So, in this case, the radio is left unsquelched and you hear any noise/hiss,
   lightening crashes, etc., along with the aircraft signals. HF is also much nosier in general than is VHF AM, so that
   adds to the noise problem. Throw into the mix atmospheric conditions, storms, etc., and it can get quite noisy on
   the HF bands. The HF feeds actually do a good job of picking up all the signals available, even with the noise
   issue.

   Hope this helps.
   Aerobill


   
   
   

[InterpreDemon]

Correct, Bill, there is virtually no atmospheric noise at VHF frequencies, which are on average twenty times higher than HF frequencies, which is why they use the SelCal (selective calling) function to send tones to un-mute the receiver of the aircraft they are calling. If pilots had to constantly listen through all that crap for a call it would drive them nuts, which it doubtless does when their SelCal is inop and they have to maintain a listening watch. You do however have to tip your hat to the ground operators who have to listen through it for their entire shift, and in the summer it will get much worse.

[jeanlottie]

Ok i understand now, i allway have this doubt.  thanks aerobill and interpredemon for the answers