Hi,

I've seen here before suggestions about using a tuned loop to increase
the gain of radio controlled clocks. Do you think this could also be
used to increase the gain of a gps receiver?

I guess that loop should be 1wl circumference, or about 2 1/2 inch
diameter, easy enought to cary in a pocket and play with.

I know one can not have more than 0 dB with full omni, I just guess the
minimalistic antenna in pocketable gps is way below 0 dB and could
maybe be improved a little.

Could this be modeled in EZNEC, one segment with some series resitance
for high loss antenna and the other 19 to simulate the loop?

Toni wrote:
Hi,

I've seen here before suggestions about using a tuned loop to increase
the gain of radio controlled clocks. Do you think this could also be
used to increase the gain of a gps receiver?

No. If you were to increase the gain of your GPS antenna, either by
redesign of the antenna or by an external parasitic structure of some
sort, it would have to result in a narrower pattern. So you'd reduce the
reception in some directions.

I guess that loop should be 1wl circumference, or about 2 1/2 inch
diameter, easy enought to cary in a pocket and play with.

I know one can not have more than 0 dB with full omni, I just guess the
minimalistic antenna in pocketable gps is way below 0 dB and could
maybe be improved a little.

0 dB relative to what?

Once you get the desired coverage angle, the only way to improve the
reception of the GPS is to improve the receiver signal/noise ratio. The
only way you can do that from outside the GPS is to use an external
antenna with a preamp having a lower noise figure than the GPS's receiver.

Could this be modeled in EZNEC, one segment with some series resitance
for high loss antenna and the other 19 to simulate the loop?

I'm not sure what the "high loss antenna" is. If you mean the GPS
antenna, it's not high loss at all, but is likely very efficient. If
it's a patch antenna, you can't model it at all with EZNEC. But even if
it's a quadrifilar helix, you can't model it with one segment.

Roy Lewallen, W7EL

On Fri, 16 Dec 2005 22:09:02 -0800, Roy Lewallen
wrote:

Toni wrote:
Hi,

I've seen here before suggestions about using a tuned loop to increase
the gain of radio controlled clocks. Do you think this could also be
used to increase the gain of a gps receiver?

No. If you were to increase the gain of your GPS antenna, either by
redesign of the antenna or by an external parasitic structure of some
sort, it would have to result in a narrower pattern. So you'd reduce the
reception in some directions.

Heh heh. This can also be beneficial... I was slightly involved with
this a few years ago:

"Raytheon’s Anti-jam GPS Receiver (AGR) supports the Tactical Tomahawk
missile program. The AGR is a PPS (i.e., Y-code) GPS receiver that
operates on both the L1 and L2 frequencies. When configured with a
multi-element Controlled Reception Pattern Antenna (CRPA), the AGR’s
post-correlation nulling techniques allow continued satellite track in
the presence of high levels of hostile jamming.

The AGR’s patented approach to anti-jam also implements satellite beam
steering, to further enhance the tracking thresholds, and to mitigate
the “spurious nulls” that can degrade the performance of other nulling
implementations (e.g., pre-correlation).

The AGR sequentially tracks up to eight visible satellites, and
provides high-quality pseudorange/delta pseudorange (PR/DPR)
measurements corrected for the effects of selective availability. The
Tactical Tomahawk’s navigation processor then uses the PR/DR
measurements to yield a high-performance navigation solution."

ref: http://www.raytheon.com/products/pgs/

Hi Roy,

Thanks for the answer. Please don't think I'm trying to be "smart",
just curious about these things and not knowledeable enough.

Roy Lewallen wrote:
Toni wrote:

I've seen here before suggestions about using a tuned loop to increase
the gain of radio controlled clocks. Do you think this could also be
used to increase the gain of a gps receiver?

No. If you were to increase the gain of your GPS antenna, either by
redesign of the antenna or by an external parasitic structure of some
sort, it would have to result in a narrower pattern. So you'd reduce the
reception in some directions.

When talking about loop antennas people talk about "capture area".
Whatever that is, this seems to be what makes a ferrite bar antenna
more sensitive than the equivalent simple coil tuned to the same freq.
I know that by using a ferrite bar you are narrowing the pattern, but
I'd think that the main gain does not come from the pattern narrowing
but from "capture area increase" (again, please bear with my ignorance,
these are only my thoughts on what I've read on the Web)

I know one can not have more than 0 dB with full omni, I just guess the
minimalistic antenna in pocketable gps is way below 0 dB and could
maybe be improved a little.

0 dB relative to what?

An isotropic antenna; AFAIK a perfect isotropic antenna would have 0 dB
gain

Once you get the desired coverage angle, the only way to improve the
reception of the GPS is to improve the receiver signal/noise ratio. The
only way you can do that from outside the GPS is to use an external
antenna with a preamp having a lower noise figure than the GPS's receiver.

Please let me doubt that. For a given coverage angle you can't make
better than a perfect antenna, but you can certainly make worst (think
of a T2FD).

I'm not sure what the "high loss antenna" is. If you mean the GPS
antenna, it's not high loss at all, but is likely very efficient. If
it's a patch antenna, you can't model it at all with EZNEC. But even if
it's a quadrifilar helix, you can't model it with one segment.

I dont know how efficient they are, but I do know that normal
commercial patch antennas are noticeably less efficient than helical
ones, and then, for the helicals, I doubt that something aprox 1/10 wl
is anything close to efficient. If this was to be true I'd love to
build an equivalent 6.5 ft. helix to work on 20m!

73s, Toni

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. But a helix and a half-wave dipole will both
be very nearly 100% efficient. The difference is that the helix is
quite directional.

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? 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?

Cheers,
Tom

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:

When talking about loop antennas people talk about "capture area".
Whatever that is, this seems to be what makes a ferrite bar antenna
more sensitive than the equivalent simple coil tuned to the same freq.
I know that by using a ferrite bar you are narrowing the pattern, but
I'd think that the main gain does not come from the pattern narrowing
but from "capture area increase" (again, please bear with my ignorance,
these are only my thoughts on what I've read on the Web)

Capture area is exactly the same thing as gain, but expressed in
different units. There are two ways of increasing the gain or capture
area of an antenna: increase its efficiency, or narrow its pattern. The
former increases the gain or capture area in all directions; the latter
increases it in some directions at the expense of others. A ferrite loop
antenna simply has better efficiency than a standard loop of the same
physical size. Hence it has better gain or capture area.

I know one can not have more than 0 dB with full omni, I just guess the
minimalistic antenna in pocketable gps is way below 0 dB and could
maybe be improved a little.


[RL]
0 dB relative to what?


An isotropic antenna; AFAIK a perfect isotropic antenna would have 0 dB
gain

dB is a ratio, in this case of gains, or field strengths with a given
power input, or capture areas. (All three ratios are the same for a
given pair of antennas.) So the reference always must be specified,
otherwise a statement of dB gain is meaningless. When using the gain of
a free space isotropic antenna as a reference, gain is expressed in dBi,
that is, dB relative to isotropic. The gain of a perfectly
omnidirectional antenna over an infinite ground plane is 3 dBi, since
the same power is concentrated in half the volume as it is for an
isotropic free-space radiator.



[RL]
Once you get the desired coverage angle, the only way to improve the
reception of the GPS is to improve the receiver signal/noise ratio. The
only way you can do that from outside the GPS is to use an external
antenna with a preamp having a lower noise figure than the GPS's receiver.


Please let me doubt that. For a given coverage angle you can't make
better than a perfect antenna, but you can certainly make worst (think
of a T2FD).

If by "perfect" you mean "perfectly efficient", I agree.


I'm not sure what the "high loss antenna" is. If you mean the GPS
antenna, it's not high loss at all, but is likely very efficient. If
it's a patch antenna, you can't model it at all with EZNEC. But even if
it's a quadrifilar helix, you can't model it with one segment.


I dont know how efficient they are, but I do know that normal
commercial patch antennas are noticeably less efficient than helical
ones, and then, for the helicals, I doubt that something aprox 1/10 wl
is anything close to efficient. If this was to be true I'd love to
build an equivalent 6.5 ft. helix to work on 20m!

I wasn't aware of that. So if typical GPS patch antennas are indeed
significantly inefficient, the reception could be improved without
narrowing the pattern. But I don't know if there would be any practical
way to do this externally.

The civilian GPS frequencies are about 1228 and 1575 MHz. The wavelength
in air of the lower frequency is about 9.6 inches. Patch antennas can be
made with a side equal to about 0.25 wavelength divided by the square
root of the dielectric constant of the material within the patch. So for
Teflon dielectric (er ~ 2.3), a patch would have sides of 1.6 inches. If
something like alumina is used (very low loss, dielectric constant ~
10), the side would be 0.76 inches. Some patches have sides twice this
long, but alumina would permit even one of those to be used in a typical
GPS receiver. The two versions of patches have different patterns, and I
don't know right off which would be preferable for GPS use. At any rate,
I believe that patches of those dimensions are quite efficient, assuming
the dielectic has low loss. Both Teflon and alumina, as well as others,
meet this criterion. Maybe someone with a greater knowledge of patch
antennas, and GPS antennas in particular, could provide some additional
information.

Roy Lewallen, W7EL

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

"Roy Lewallen" wrote
A ferrite loop
antenna simply has better efficiency than a standard loop of the
same
physical size. Hence it has better gain or capture area.
=========================================

It's only a minor point, but when a ferrite core is placed inside a
loop the efficiency remains the same. It's the same wire, the same
coil dimensions, and hence the same loss in the resistance.

If anything happens to efficiency it is reduced due to a loss in the
ferrite core material.

What happens is that the effective cross-sectional area of the loop
increases approximately in proportion to the permeability of the core.
For small permeabilities the capture area is much increased. But for
larger permeabilities, say above 100, the effect diminishes and the
effective core permeability settles down to the order of 20 or 30. It
depends on the ratio of length to diameter of the core rather than of
the coil.

To visualise, it should be remembered most of the magnetic circuit
lies in the air between and near the ends of a ferrite rod. There is
no point in increasing permeability of rod material beyond a certain
amount in an attempt to increase capture area.

Capture area depends on the volume of the region of the e.m. field
surrounding the core which is distorted by the core. See iron filings
sprinkled round a bar magnet. When the reluctance of the rod itself
is zero compared with that of the air path the limit of permeability
has already been reached.

And high values of rod permeability are always associated with high
core loss. The moral is obvious.

The sensitivity of the coil itself to e.m. waves is calculable. I'll
stick out my neck and say the best a ferrite core can do is to
increase the effective diameter of the coil by crudely 5 or 6 times.

Which means that the gain of your tiny pocket MF transistor radio is
much better than your super-dooper transceiver which needs a 260-feet
long dipole on the 160 meter band.

Just for the sake of argument!
----
Reg.