[time-nuts] Monitoring the Rapco 1804M - how antenna changes affect the unit

Dave Martindale dave.martindale at gmail.com
Wed Nov 2 12:55:37 EDT 2011

By the way, this doesn't mean that the GA-27 is a poor antenna design,
it's just not the best antenna choice for this situation.

The GA-27 is intended as an external antenna for Garmin's handheld
receivers, which normally operate with passive patch or helix
antennas.  So the receiver itself needs to be sensitive enough to work
well when an unamplified antenna is connected directly to the
receiver, and the GA-27 preamp only needs to provide enough gain to
overcome the losses in the cable plus a bit more.  Additional gain
could be detrimental in some circumstances (e.g. when placed near
transmitting antennas on a vehicle), so the GA-27 might well be the
best antenna for the Garmin handheld receivers.

But a GPS board module is intended to be built into some piece of
equipment, and probably expected to always be fed from a remote
amplified antenna, never a local passive antenna.  In those
circumstances, it makes sense to put all the necessary gain at the
antenna preamp and let the receiver be less sensitive.  The
manufacturer will recommend some antenna for use with the receiver,
and though you don't usually have to use that particular antenna, it's
probably a good idea for your antenna/cable/splitter setup to provide
roughly the same signal level at the receiver.

- Dave

On Wed, Nov 2, 2011 at 01:19, David J Taylor
<david-taylor at blueyonder.co.uk> wrote:
>> I've made a small Web page describing what happened when I changed the
>> puck antenna on a Rapco 1804M for a more sensitive one.  The Rapco 1804M
>> expects a big outside antenna.
>>  http://www.satsignal.eu/ntp/Rapco-1804M-notes.html
> This page has now been updated to provide a comparison between three puck
> antennas - the Garmin GA 27 (BNC), a 3rd-party low-cost puck, and a Gilsson
> puck antenna.  It seems that the even higher signal level from the Gilsson
> benefits the 1804M more than the other two.
> Cheers,
> David
> --

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