Right now I am looking at modifying a unit that receives 850, 900
frequencies.
What I am curious about is the concept of small antennas only in
reverse.
A small antenna might be a autombile antenna used for AM reception.

So, taking the unit to receive 850, 900 MHz freuquences, how well
would it receive 2.4 GHZ frequences?
It would seem that it would receive 2.4 GHZ well due to the concept of
effective length but I am not sure.

Any comments?

Thanks, Dave

"EE123" wrote in message
...

Right now I am looking at modifying a unit that receives 850, 900
frequencies.
What I am curious about is the concept of small antennas only in
reverse.
A small antenna might be a autombile antenna used for AM reception.

So, taking the unit to receive 850, 900 MHz freuquences, how well
would it receive 2.4 GHZ frequences?
It would seem that it would receive 2.4 GHZ well due to the concept of
effective length but I am not sure.

Any comments?

Thanks, Dave


Dave

While it is true that the antenna will be several wavelengths long and will
work reasonably well for receiving there are other factors to take into
consideration. As frequencies get higher, signal levels are generally at
lower levels. There isn't so much atmospheric noise to overcome and at
frequencies above VHF, most communications are line of sight.

For this reason transmitter powers are generally much lower and directional
antennae are used as the cheapest way of boosting signal strengths. Log
periodic and yagi antenna designs are among the best known types of
directional antennae, but other designs can be used very effectively.

Your AM automobile antenna receiving pattern will be broken up into a series
of lobes due to its being several wavelengths long at 850-900 MHz. The
performance in any given direction will be a bit of a lottery. Have a look
at the EZNEC program to get some idea of how the transmitting and receiving
patterns of antennae change with frequency and multiple wavelengths of
antenna.

For all round (omni directional) reception, the height the antenna is
mounted is more critical than anything else. Whatever design you decide
upon, get your antenna as high as possible and connect it to the receiver
using good quality, low loss cable. Satellite cable will do at a pinch. Even
a simple dipole antenna mounted vertically and clear of obstructions around
30 feet up should provide a host of signals. The dipole elements only need
to be around 6 inches long. That should give you an idea of what signals are
about and you can progress from there.

Mike G0ULI

EE123 wrote:
So, taking the unit to receive 850, 900 MHz freuquences, how well
would it receive 2.4 GHZ frequences?

If it is vertical and omnidirectional, it will probably
become an end-fire antenna at the higher frequencies.
That means it will likely be directional but mostly
in an up direction. The same thing happens when
a 19 inch vertical whip is used on 440 MHz.
--
73, Cecil http://www.w5dxp.com