Hi Tom,

K7ITM wrote:
Looks to me like you are confusing "efficiency" with "directionality".

That is, a small loop, or a short dipole, will have a low radiation
resistance, and when it's small or short enough, the resistance in the
conductors of the antenna becomes appreciable compared with the
radiation resistance. That causes a loss of efficiency, because the
signal energy ends up heating the wire resistance instead of being
useful to the receiver.

No, I was not confussing it. When I talk about "efficiency" I amb
refering exactly to that, to radiated energy vs. energy lost heating
the wire.

But a helix and a half-wave dipole will both be very nearly 100%
efficient.

There are many classes of helix antennas, and all the "normal" helix
antennas you will find have dimensions comparable to a full wl if not
greater.

My doubting on pocketable GPSs antenna's efficiency is based merely on
their size. They are about 1/10 wl long. The only other antennas of
comparable size I know are loops, isotrons, fractals, CFA, EH and CB
sticks and, except for tuned loops, we know how they are treated
whenever they appear in this forum.

The difference is that the helix is quite directional.

The ones used in GPS receivers are exactly the opposite. They should
ideally have a "half orange" radiation pattern, as they should be able
to follow a satellite from horizon to horizon all over your head.


One way to get better reception would be to have a set of highly
directional antennas (high gain antennas) that track a set of the
visible satellites. Do you really want to do that?

No, of course not

Or do you perhaps
instead want an antenna that has a pattern that "sees" better in a cone
with maybe a 25-40 degree elevation above the horizon, because the
satellites directly overhead are not generally the problem? As is
commonly the case, you should probably consider the whole system, not
just one part of it. What, exactly, is the goal?

The goal is improving the efficiency, while not reducing (much) the
directionality of a small antenna inside a plastic case that can not be
modified.

Before you say this is impossible please think it twice:

Think of the small coupling loop used in a tuned loop. By itself it is
a very bad antenna, but when an appropiate parasitic structure is added
(the whole tuned loop) the "whole system" becomes a decent antenna.
This is exactly what I'm asking about, about using my GPS antenna as a
coupling to a more efficient structure of some kind.

Haven't you tried to place a walky close to a full size 2m dipole? When
both antennas are paralel and very close to each other it certainly
improves the signal strength.

Cheers,
Tom

Cheers and thaks for your answer

Toni

Toni wrote in part:
Hi Tom,

K7ITM wrote:



The difference is that the helix is quite directional.


The ones used in GPS receivers are exactly the opposite. They should
ideally have a "half orange" radiation pattern, as they should be able
to follow a satellite from horizon to horizon all over your head.

Isn't that quite directional?

And are you talking about helical as in a helical antenna, or are you
talking about rubber ducks?


Before you say this is impossible please think it twice:

Think of the small coupling loop used in a tuned loop. By itself it is
a very bad antenna, but when an appropiate parasitic structure is added
(the whole tuned loop) the "whole system" becomes a decent antenna.
This is exactly what I'm asking about, about using my GPS antenna as a
coupling to a more efficient structure of some kind.


It sounds as if you are proposing something that will be quite
beneficial to the radio world if you succeed.

- 73 de Mike KB3EIA -

Toni wrote:
. . .
There are many classes of helix antennas, and all the "normal" helix
antennas you will find have dimensions comparable to a full wl if not
greater.

Quadrifilar helices typically have four twisted half wave elements. The
length for 1228 MHz would be somewhat less than 4.8 inches. Twisting
would make the assembly less high that that, and fattening the elements
or plating them on a dielectric substrate would further shorten them.
This is consistent with the antenna of an older GPS unit I had.

My doubting on pocketable GPSs antenna's efficiency is based merely on
their size. They are about 1/10 wl long. The only other antennas of
comparable size I know are loops, isotrons, fractals, CFA, EH and CB
sticks and, except for tuned loops, we know how they are treated
whenever they appear in this forum.

It's possible to make an electrically small antenna that's quite
efficient. Typical examples are the small transmitting loops made by AEA
and MFJ, or the shortened, top loaded verticals described by Jerry
Sevick, W2FMI, in a series of articles in the '70s. What you can't make
is a short, efficient, broadband antenna. But GPS antennas don't need to
be broadband. And for that matter, they don't have to be that
electrically small.


The difference is that the helix is quite directional.


The ones used in GPS receivers are exactly the opposite. They should
ideally have a "half orange" radiation pattern, as they should be able
to follow a satellite from horizon to horizon all over your head.

This describes the approximate pattern of both quadrifilar helix and
patch antennas, which is why those are the types which are commonly used.

. . .

Roy Lewallen, W7EL