External Antenna for GTM 20?

For those who have an older model GTM-20, the new model has a standard lighter plug and the receiver is molded into the cable a few inches from where it plugs into the nüvi's cradle. It has a provision to plug an external antenna into it, presumably for better FM reception, but Garmin doesn't seem to know about it.

I sent an email to Garmin to ask about it and the 1st reply was a stock answer on how to plug the cable into the lighter socket and the cradle. I replied with my original note and useless response asking for info specifically for the external antenna on the GTM-20 with the following reply:

Normally Garmin is much better than this!

Now that kcgpsfan is also interested, maybe one of our fellow readers may have an answer. It looks like a length of wire with an appropriate plug would be all that is needed and easy to DIY. Knowing which plug to get from Radio Shack might be the hardest part. Any thoughts?

FYI: I have my GTM-20 plugged into a power point near the floor on the rear of the console with the cable running along the floor between the front seat and console and the nüvi is mounted low on the console beside me knee. I was surprised that traffic reception worked well in this arrangement. An external antenna may extend the useful range of the traffic info, but it works so well this way I didn't pursue the antenna with Garmin any further.

The smallest plug that Radio Shack sells is 3/32 plug. Haven't found out if that is the one needed.

.

The length of the wire is the most important part of the antenna. You need to know the frequency of the xmitted FM signal. From that you can determine the wave length in inches. The ideal length would be 1 wave length long. Any thing less will cut down on the signal strength. and if it is too short such as a quarter wave length long it could/will act as a short.

Just adding some grist to the subject......

I'm not a radio tech, but does it have to be that complicated? As I understand it, the traffic data piggybacks on existing commercial FM broadcast station signals. The FM band lies in the gap between the analog TV channels 6 and 7. Many portable FM radios and TV sets use a simple telescoping rod antenna with a single wire connecting it to the tuner. From a transmitting standpoint, I agree that the the design of the antenna is critical, but for a receiver, is that necessarily so?

Essentially an antenna is a portion of a tuned circuit that will resonate when excited by electromagnetic waves from a transmission source. The length of an antenna determines its resonant frequency which is as you stated much more important when transmitting than receiving. When receiving, the more wire the better chance to capture the required signal. For a simple plain wire antenna for FM, a piece of plain wire about 30 inches long is a 1/4 wavelength antenna.

The Radio Shack 3/32 mono plug seems to fit. (cat #274-0289). Now the qestion is, which plug teminal hooks to the piece of wire? Or can you get a flat wire FM antenna and connect the two wirs to each of the plug lugs?

Normally for a two wire plug/receptacle, the center pin or tip is the active conductor and the sleeve or ring is the shield.

If you have an FM antenna from a Sirius Radio, that should work. I have my Sirius receiver connected to my AUX input, so I don't use the FM transmitter antenna. It uses the same jack as the Garmin.

Thanks for the info. We wouldn't be lucky enough to have a secret dianostic screen to monitor the signal strength for the traffic receiver? It would be handy to check how good the signal is when experimenting with various antenna lengths.
I never heard of one, but one can ask I guess.

Oops, it pays to read the manual. It shows on page 2 how to read the signal strength. It's in the top right corner

Signal strength is available as a numerical value (rather than signal strength bars) in the GTM status page:

From the trip computer screen (with speedo dial), press and hold the speedo dial for ~10 seconds. Advance through the diagnostic pages til you hit the GTM Status page.

Thanks. I was just going to write that I reviewed the manual online for the GTM 10, not the 20. I thought I was out of luck.
I hooked up a strand of 30 inch wire pair, each wire soldered to a lug. At the end, the wires are not connected. I don't know if they're suppose to be to create a loop. I'll have to experiment now to find out if it affects the reception.

The old UHF antenna was a simple loop, but that is in the UHF band. I would experiment, try the single lead connected to the center pin as a_user suggested. then try 2 equal length leads, one to each pin and laid out in a line, like rabbit ears when set out flat. Maybe a HAM might chime in on the loop idea before trying that.

My early thoughts were to lay this out along the edge of the windshield concealed in the plastic trim to possibly improve FM reception.

We await your results.

I tried 30 inches of double strand wire, with each wire hooked to the lugs. Using the signal strength indicator (thanks to an earlier post), I found that my efforts did not produce better results. The signal before the antenna was at its peak at 42 and when I hook up the antenna it was down to 16 to 20. I tried hooking the two wires at the end together to see if a loop was better, but it didn't help. Now I could try just one length of 30 inch wire, then try adding to the length. I don't know if the thickness of the wire has anything to do with it. What I'm using now is quite thin. I don't think the flat fm antenna wires are very thick.
All these efforts may be mute, since I haven't traveled with the receiver to see how the reception is in certain areas. Still it would be nice to construct something that would help in fringe areas.

If you remember what that old flat 300 ohm twin lead tv antenna wire looked like... I think that was about #18 or #20, just a guess, a bit thinner than a standard wood pencil lead. Within reason, I doubt the wire gauge on a receiving antenna makes much difference.

The power cable on the new GTM-20 looks like it may have the antenna lead running the length back to the lighter plug. You said the signal dropped when you plugged the homemade antenna in... That may indicate that inserting the plug disconnects the built-in antenna.

-Or-

Is the signal strength measured in dB? Is it 42dB or -42dB? I don't have my nuvi handy to look at that screen. I seem to think that -20dB is stronger than -42dB. I'll ask my HAM buddy about this later today.

If I remember correctly, it's actually around 20 to 24 guage

If it is in dB, then the smaller value is the stronger signal. A 3 dB change either doubles or halves the strength. Going from -40 to -37 means twice the signal strength.