Hi andy- thanks for the education but I'm using narrow fm for the phone downlink , not packet. I did try a diplexer with a .25 db loss between the antenna and the mast mounted preamp but noticed significantly lower recieve at or near fringe. I'm in a rural area. Big power stations are not a problem. Thanks for info, pat
On Sat, 26 Aug 2006, Andrew T. Flowers, K0SM wrote:
McGrane wrote:
the radio set in the Narrow FM mode for greater sensitivity.
Two things come to mind, and I want to make sure anyone reading this thread doesn't get confused:
The first is to make sure that the NBFM bandwidth is wide enough to pass 9600 baud transmission. Doesn't one need about 20 KHz? I'm sure someone out there knows. Don't forget that you have to account for being off frequency due to doppler, particularly if you are using 5-kHz steps.
Secondly, the mode setting on the radio is something that affects a filter in the IF chain, not the front-end of the radio. I think the issue with the FT-8800--and generally any other transceiver designed to double as a wider VHF receiver--is that the the front end amplifier is overloaded by a strong signal at *any frequency*. The classic receiver design has a (preferably low-noise) amplifier as the first thing in the receive chain, which boosts the incoming signal enough to overcome the following mixer losses. That ampflifier is being bombarded by everything from DC-GHZ, and will amplify whatever the transistor is capable of. (There is usually some sort of filter ahead of it in commercial equipment, but in the case of wide-band VHF receiver it's likely to be pretty wide).
Now as we all know, amplifiers have a limit as to how much output they can supply, and after an incoming signal gets too loud it will become distorted. This is familiar to many of us in urban areas when we suddenly hear a pager on the frequency of our favorite repeater. That is often an effect of that very strong signal being clipped and introducing new frequency components. Another effect is that a strong incoming signal to that front end--not matter what frequency it is on--effectivly swamps the amplifier so that the weaker signals one desires to listen to are not amplified faithfully. This often manifests itself as "deafness", but it is really because the front end is spending what juice it can supply on that big signal 50 MHz away from where you are trying to listen. No changes to IF filters are going to make that signal go away. When this happens one usually has to use some sort of a filter ahead of the amplifier, either to notch out the offender, or pass only the band of interest. This will allow the front-end amplifier to run in its linear region, and thus amplify all the incoming signals without (significant) distortion.
The lab specifications for front-end overload can generally be figured from the thrid-order intercept point (IP3), which essentially tell you how much distortion one gets for an input signal of a given amplitude. The "sensitivity figure" doesn't tell you anything about how the radio will perform in a high-RF environment.
I don't mean to lecture, but rather clear up some possible confusion.
-Andy K0SM/2
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