REVIEW COMMENTS: Re-Radiating Antennas

steve.dawes

Please can you give details of installation ?

Thanks

Darren

It depends on what re-rad you are looking at, most come with an external receiver antenna which goes outside and a transmitter which needs to be within 45cm or so of the receiver (your PPC/GPS). GPSW's RRAD-45 is such a device but the lead to the internal Tx antenna is only 1m long.

There are other more powerful Re-Rads that radiate the signal 2-3m but these bring their own problems with over saturation of the signal drowning your GPS receiver etc. These are intended for installations where the GPS receiver could not otherwise receive any signal at all. See GPSW's RRAD-46.
_________________
Darren Griffin

forumhound

I have a Rav4 with a (cigar lighter) power socket in the boot. If I plug the re-rad aerial into the socket, place the antenna out through the rear door, then place my 4400 navman up front with me will I gain anyting (ie, will the 4400 be too far away from the re-rad to obtain a better signal...)

Thanks

_________________
Forumhound

martynharrison

Another tip for the more powerful rerads and them in general, the most common problems created are:

- GPS receives signals from the sky, and also from the rerad. Since these are slightly out of phase, the rerad can make things worse if the problem is low signal strength. The symptom for this is poor signal strength, which drops to zero as soon as the rad is turned on, and goes back to normal full strength when you move the GPS well away from the vehicle.

- Rerad antenna receives signals from the sky, and also from the retransmitting antenna (this is far more common with the high strength rerads). What happens is the signal is in a loop so that the retransmitter is sending out a continual stream of the same message which is not more use than noise. Similar symptoms to the one above

You can often solve the loop problem by isolating the antenna from the car, in an extreme case you put a thick metal sheet on the roof and put the antenna on top of that (and then you see what else can be done which is a bit more convenient).

The "twice as many signals" problem can be solved by isolating the antenna on the GPS so it only receives from the retransmitter, e.g. by putting a metal cone over the receiver and the rerad transmitter. Or moving the unit so it isn't able to see the sky, or by positioning the rerad so it can pick up satellites from the rear of the car only, and the GPS can pick up satellites from the front only.

Having measured the 5 meter rerad range: the 45cm one is best put right next to the navman antenna, maybe ok up to a foot distant but that's low signal territory. The RAM mounting brackets (very very much better than the one the Navman comes with and suitable for ipaqs with all sorts of sleeves) has enough room to put the rerad antenna from the 45cm version in the back. This is generally ideal.

By contrast, the 3-5 meter one is the other extreme, while the range varies from one unit to the next, some are running a very high current and have ranges that exceed ten meters (the maximum length I could get in the lab) passing through thin walls quite happily. Some are more like 5m, but I got the impression that 5m was the minimum rather than the maximum. Because this is a powerful signal, it can also pass through the roof of a car and repeat itself back to the receiving antenna, creating a feedback loop.

Testing this is best done with something like a Garmin where the signal strengths are well displayed, or by capturing the NMEA sentences which include a strength meter rating (I forget which but it's easy enough to spot).

Hope this was useful.

Dave

Thanks Tim, You are correct, I've amended the review now to "no more than 45cm away from your GPS receiver".

Tim Buxton

Dave,

Your review mentions putting the re-radiating antenna about 45cm from the GPS device. The 45cm is a *maximum* distance, as that is the range of the re-radiator. Personally, I've found that the closer I position the re-rad to my Navman, the better.

_________________
Tim

Dave

Unless you have a car with a rear engine

TheBoyGroucho

Apparently a good (permanent) place to fit GPS antennas is in the plastic rear bumper...goodish visibility and a long way from the cars engine electronics, might be a good place for the re-radiating type of antenna, maybe taking the 12v feed from the rear light cluster?

Tredders

Dave- I have some timings on the TTFF with and without the re-radiating antenna. Do you want the findings emailing to you, or posted here?

Mark.

MickyFinn

I bought the more powerful model with a 3-5metre range, the downside of this is that you cannot keep the antenna inside your car as it is in the 5m range and confuses the GPS. With the antenna on the roof I got a constant 6-8 satellites and my Geodesy speed trap detector could work if placed on the floor of the car rather than the dash.

I'm going to return the more powerful antenna for the 45cm version that features in your test so that my Navman can be attached to the windscreen and the antenna for the re radiating unit can sit on the rear parcel shelf that way I should get a good view of the sky and stop vandals stealing my antenna and theives realising that there is something worth stealing.

Do you guys have the antenna on your roof or rear parcel shelf?

Thanks

Micky

Darren

It's not a signal amplifier just a re-radiator. i.e. it receives the external signal and re-broadcasts it inside the vehicle. This will improve signal quality as without it the Navman has to deal with signal degradation as the signal comes into the vehicle and the unobstructed view the external antenna has allows a much more reliable lock.
_________________
Darren Griffin

adfrad

Does the re-radiating antenna amplify the signal, or does it just funnel the signals at the identical strength from "outside" to "inside"?

A good test would be to use the GPS outdoors, and take a reading, then use the re-radiating antenna outdoors alongside it, and see if the signal quality has improved.

Tredders

No problem - I'll start taking some timings regarding TTFF over the next week (with & without the antenna), and post up the findings.

As a totally unscientific first test, I tried to get a fix without the antenna for the first time last night (using my new upgraded Smart ST software), and during a ten minute journey, never got a fix. To be honest, this is pretty consistent with my TTFF experiences with the old software, prior to buying the antenna. It was not uncommon to wait 15 minutes to get a fix, and then lose the fix during a journey.

Since having the antenna, my old Smartpath software (and also the Smart ST software) typically gets a fix within 15 seconds. Also, I rarely see less than 8 satellites locked at any one time, in contrast with the 3 or 4 I used to get.

Will post some stats in about a week.

Mark.

Dave

Navman do suggest that the TTFF problems people experience with the Navman sleeve is due to Electromagnetic interference, however signal strength does play a big part. If the signal it is receiving appears stronger to the receiver, it may be enough to overcome any interference. I haven't managed to test this myself, so I can't say for sure if it will fix it or not. It will be interesting to see what Mark's TTFF's are over a period of a week or two.

Mark, would you be prepared to create a TTFF sheet you could keep with you in-car ? Do you have a stop watch on your wrist watch that you could do timings from ?

Navm

Mark,

I am very happy for you that you are getting good results.

However I am now a little confuse as in one of articles here it was concluded that Navman takes a long time to TTF because of electrical interferance which it gets from IPAQ.

So i thought buying a re-radiating antena will not improve performance and went for TOMTOM, humm

many thanks and sorry for my poor english.

	PocketGPS
	Web