Quad-helix Vs Patch Antenna Is One Better?

[+tirediron]

The quad-heli vs. patch antenna is a topic that has arisine many times here, with lots of pros and cons being raised for each. Having recently purchased a GPSMap 76CS with the quad-helix AE, I decided to conduct a comparison against my eTrex Legend. This is not a scientific experiment, but I have attempted to make it as controlled as possible. I'm not making any statements one way or the other... read on and conclude what you will!

 

The purpose: To make practical observations on the relative merits of the Legend’s ‘patch’ antenna, and the GPSMap’s Quad-Helix as well as the gain (if any) produced by the external antenna.

 

The equipment: A Garmin eTrex Legend, a Garmin GPSMap 76CS, and an external antenna.

 

Setting: Rural yard, approximately 60 degrees of clear sky view, tall trees and buildings to the left and north, open space to south and east. Further tests were conducted under moderate and heavy tree cover. Sky was overcast, with occasional light showers.

 

Method: Initially the two units were placed side by side in their optimum position (Legend horizontally oriented and the 76CS vertically). They were given fifteen minutes to “acclimate” during which time no observations were made.

 

At the end of the initial fifteen minutes, the units were switched off for approximately 20 minutes. They were then switched on and their time to acquire was noted. The 76CS aquired in 9 seconds and produced an EPE of 41 feet. The Legend required 22 seconds and displayed an EPE of 152 feet.

 

After approximately 30 seconds the results were as indicated in the photograph:

 

 

One minute later, both were observed again, and had settled as shown:

 

 

The differences are minimal in these circumstance and while the Quad AE is displaying a marginally stronger signal, and slightly smaller EPE, it doesn’t really recommend on over the other. To test the external antenna’s capabilities with the 76CS, I moved to an area with a fairly limited view of the sky, buildings on two sides and high trees all around. This picture was taken after the unit having been powered up for about 3 minutes in the new location (<100’ from the first tests):

 

 

This picture was taken after having had the external AE plugged in for 30 seconds. The difference is quite dramatic!

 

 

Moving under moderate tree cover (Sky 75% obscured by alder trees between 25 and 40 feet tall), the 76CS Held one more satellite than the Legend and indicated an EPE of 34 feet vice 51. Moving under heavy tree cover (Dense, low-hanging bamboo, alder and fir trees) the 76CS degraded to an EPE of 53 feet and was holding alternating between two and three satellites held between the second and third divisions on the strength display. The legend held lock for a short time, with two satellites and an EPE of 150+ feet and then lost lock completely. Connecting the external AE to the 76CS again made a significant improvement, reducing the EPE to <40 feet and acquiring one more satellite as will as increasing the relative strength of all satellites.

 

Conclusion: In all but heavy cover, both units performed satisfactorily, with the 76CS/Quad antenna turning in a marginally better performance. The heavier the overhead cover, the more significant the difference. Is this difference alone worth the additional cost of a unit with the Quad-helix AE? That’s a personal decision, but I am very happy with mine! As well, for <$25.00, the external antenna was an excellent purchase, turning in better than expected results.

 

Edit: Stupid typos

Edited September 14, 2004 by tirediron

[+Searching_ut]

Good observations, but may I suggest maybe one error in the conclusion. In comparing the etrex and 76, you're comparing two different receivers that happen to have different antenna types, not different antennas. In comparing the 76 reception with it's internal quad helix antenna vs with the external patch, you're doing a comparison where everything is equal except for the antenna design....

 

This has been my issue with the "antenna" debate for quite some time.....

[+tirediron]

Good point!

[+tirediron]

I guess a better way to put it would be as a comparison of patch equipped rcvrs vs. quad-helix equipped rcvrs...

[+NightPilot]

Also, you're not putting the Legend in the optimal orientation for receiving signals. The patch antenna of the Legend works better if it's held flat, with the antenna facing the sky. It will work held vertically as you show, but the signal strength won't be optimal, and it won't pick up satellites behind it very well. Put the Legend on top of that chunk of wood, lying flat, and I'll bet you get a better signal and faster acquisition.

[peter]

I guess a better way to put it would be as a comparison of patch equipped rcvrs vs. quad-helix equipped rcvrs...

No, a better way to put it would be as a comparison of one 76cs vs. one eTrex Legend under a specific set of conditions.

 

Whether the antenna type had anything to do with the results can't be determined without a controlled experiment where you use the same receiver and only vary the antennas. Your second experiment comparing the 76cs with the internal quad antenna vs. the external patch comes close to this but it's not clear if the external patch was placed in a better orientation and position - the picture shows the 76cs lying flat for this test which would not be optimal for the internal antenna.

[+tirediron]

Also, you're not putting the Legend in the optimal orientation for receiving signals. The patch antenna of the Legend works better if it's held flat, with the antenna facing the sky. It will work held vertically as you show, but the signal strength won't be optimal, and it won't pick up satellites behind it very well. Put the Legend on top of that chunk of wood, lying flat, and I'll bet you get a better signal and faster acquisition.

The orientation is only for the pictures. They spent all their time in their optimal orientation. Note from my OP:

 

Method: Initially the two units were placed side by side in their optimum position (Legend horizontally oriented and the 76CS vertically). 

 

With respect to Peter's comments, I did say this was NOT a scientific experiment, rather observations, still, I agree with your comments. Again the same applies to orientation.

[+EraSeek]

I have read that it is not really so much the type of antenna as the size of the antenna. The quad helix being larger than the patch in the etrex line naturally will do a bit better.

 

I have a question. Isn't external antennas really just larger patches? or is there another atribute to them that will improve reception?

 

I've also read a study where an external antenna does not improve things with open sky, but does improve things in a forested situation. I guess my question would be why?

[peter]

Yes, most external antennas are of the patch design and tend to be a little larger than the internal ones. The main benefit is by allowing you to place the antenna in an optimal location and orientation and still leave the receiver in a convenient spot for carrying or seeing the screen. The external designs usually include a low-noice preamp to compensate for the signal losses in the long coax cable while the internal antennas feed directly into the receiver's amplifier.

 

I'm not surprised that there was no improvement under an open sky since the unit's internal antenna would normally be able to get an adequate signal from all visible satellites in that situation. The main advantage for the external antenna comes when many of the satellites are blocked from view of the internal antenna - by foliage, portions of the user's body, car roof and/or windshield coating, etc.

 

The many complaints about reception by the original eTrex series (ie. not the 'C') are rather puzzling. Garmin's tech support has indicated that the antennas are the same size and design as those used in the 12 series and eMap yet those units saw few complaints. And there is considerable sample-to-sample variation among the eTrexes.

[+GOT GPS?]

Things that I can think of with the eTrexes:

I noticed with my Vista that the thumb over the click stick could block signal slightly, and also if the Vista was allowed to dangle around my neck for even a short while, then it did poorly. If i am lazy the Vista may loose it's horizontal angle if i let go of it for a bit. With my other GPSs, if i let them dangle for a bit, they are still in their vertical orientation, and they don't loose too much signal. One thing ive noticed, is that Pine trees can kill any signal, to even a good GPS. In my front yard, my eTrex Vista never would get a signal because of the huge trees, but my other GPS's didn't have too much problem.

Edited September 15, 2004 by GOT GPS?

[+tirediron]

To add on to what GOTGPS said, I think that a common problem with the eTrex line is that they tend to often be held in a "body shadow". I have noticed that when I hold the unit at arm's length totally clear of my body, there is usually a marked improvement in reception. Unfortunately, it's hardly practical to walk around like this for any period of time. Depending on the way you hold it, and the position of your body relative to the view of the sky, and the satellites, this could attenuate a fair amount of the signal.

 

With the 76CS I notice that I tend to carry it higher, and thus it is less "shadowed" by my body. I don't know if this applies to others or not...

[+EraSeek]

The many complaints about reception by the original eTrex series (ie. not the 'C') are rather puzzling. Garmin's tech support has indicated that the antennas are the same size and design as those used in the 12 series and eMap yet those units saw few complaints. And there is considerable sample-to-sample variation among the eTrexes.

It is likely a bias against size. I would never buy a geko! The thing just can't be real!

 

I have also seen studys where patchs performed quite well, even though everyone thinks they are worse than the quad helix. I'd like to look those studies up again and review them.

[+PDOP's]

My first GPSr was a GPS12 and I now have a GPS76. I don't find that the 76 performs much better than the 12 but I have noticed that the 76 is far more sensitive to orienataion. My old 12 was happy working horizonatlly as would be expected with the patch antenna and worked equally as well vertically in my car. The 76 has trouble maintaining a lock unless it's held vertically.

[+EraSeek]

My first GPSr was a GPS12 and I now have a GPS76. I don't find that the 76 performs much better than the 12 but I have noticed that the 76 is far more sensitive to orienataion. My old 12 was happy working horizonatlly as would be expected with the patch antenna and worked equally as well vertically in my car. The 76 has trouble maintaining a lock unless it's held vertically.

Exactly the same here, only I have a 12 and a MAP76. I still have both.

[+GOT GPS?]

How about this one:

 

The Legend 60C

 

Could hold it anywhere from horizontal to vertical, and with a patch and a helix antennas, it would be perfect.

 

Now if it had Dual 12-channel recievers, one for each antenna, that would help the accuracy alot, and you would not get the run-around at the cache site either.

Edited September 15, 2004 by GOT GPS?

[+PDOP's]

Might be a winner if it had FRS/GMRS radios too.

[+tirediron]

Just a second while I mortgage my house and sacrifice my first-born to pay for one...

[+Searching_ut]

I have also seen studys where patchs performed quite well, even though everyone thinks they are worse than the quad helix. I'd like to look those studies up again and review them.

 

There used to be quite a few accuracy studies you could find on the web, and even a few that attempted to measure sensitivity. Virtually all of them show the Patch antennas to have better accuracy in the trees, and more often than not also show the quad helix does slightly better in clear skys.

 

Some of the ones you can still find:

 

http://www.fs.fed.us/database/gps/mtdc/gps.../Nav_3-2001.htm

 

Note when sticking to same manufacture, the GIII does noticeably better in clear skys, the etrex is more accurate in the trees. Also note that with magellan, the patch antenna 2000 has similar results when compared to the blazer.

 

http://www.fs.fed.us/database/gps/mtdc/map...smap_76_rev.pdf

 

Note in this test that when using the internal antenna vs a patch external, accuracy was about the same in clear skys, noticeable improvement in the trees.

 

My own observations, especially with dynamic side by side comparisons (Reading while moving) I found the accuracy differences to be very noticeable, with the quad antennas wandering and getting lost considerably more than the patch antenna receivers when the reception conditions got bad.

 

As for the eTrex line, there seems to be a lot of differences with the receiver that I think account for the "reception" issues people observe. They're very fussy about orientation when compared to most units, with perfectly flat not always being what they like. They are also very fussy about where you hold your hand, how you hold it (ie. waist level flat, tucked into the belt not a good choice but what tends to come natural). The software in the eTrex line seems to be different as well, with things like the way they reacquire after a lock being different. They often take a long time to get a good fix again, even when you get back into a clear sky condition. Interestingly, with the New c series eTrex's, the software, and quite possibly the processor etc seem to be the same as the 60 series, which may account for what makes it seem to behave better in the crud.

 

As for differences in general, it's hard to say when comparing different units. Are the electronics different resulting in different noise levels? Is one antenna better than the other? Are the differences a result of different ways of processing and displaying the results, which may or may not cause different behavior in different conditions. Isolating the actual parts that result in the differences is tough being as how you can't isolate the different components.

[+Sputnik 57]

I have a question. Isn't external antennas really just larger patches? or is there another atribute to them that will improve reception?

 

I've also read a study where an external antenna does not improve things with open sky, but does improve things in a forested situation. I guess my question would be why?

Here is a study comparing 4 external antenna types. Not an exact answer to your question, but interesting.

[Dukie 'n' Dad]

You also need to take into consideration the electronics and software. I was amazed recently when i put my new Vista C next to a new Legend that the Vista C was able to get a good signal while the Legend was significantly lower. Still had roughly the same 'fix', but the signal strengths were vastly different. Of course, that could be yet another software thing, but from a bunch of observation of the two units, iit sure appeared that the Vista C could simply 'hear' the signals better. In maginal conditions, the Legend would lose it's reception while the Vista still had a small bar showing.

 

In the field, we have not noticed any real differences. Of course, Dukie is color blind, so he is not swayed by sexy color images.