UM150 Problem - Homemade Antenna Debacle

Derjyn · EVDO Newbie Joined: 24 May 2008 Posts: 7

Greetings,

I've been pretty happy with my UM150/Verizon provider. I was previously getting around 1000kbps - 1500kbps DL, and 250kbps - 400kbps UL, with average ping times of 120ms. Though my RSSI was always around -90/100 dBm... So I decided to try and fashion a quick homemade antenna to jack into the back of my UM150- just to see.

I'm no hardware guru, but I can fiddle a bit. So I grabbed a thumbtack, radio antenna, and some speaker wire and got to work. I'm sure you can see where this is going... It worked fine, the thumbtack fit snug as a bug into the antenna port on the back of the UM150, with the speaker wire wrapped around the metal pin of the tack. Moving the antenna around, I got my RSSI to around an average of -85/95.

Definitely not a marked improvement, as I noticed no increase in speed. Now the problem comes when I removed the "antenna" and put things back the way I had them previously- I had a permanent home for the UM150. Now, for some reason, unless I have my jimmy-rigged antenna "plugged" in, I can't get any signal!

Is the UM150 stuck in some exterior antenna mode? I've removed the drivers, VZAccess Manager, everything related to the UM150 and started over again, re-installing from scratch. Same thing, no go without the antenna slapped in there...

Now before you go saying I broke the little guy, and I need to get a new one, I have to say that I was very cautious while doing this. I made sure not to push too hard and break through the port... So I'm very sure that I didn't bust it. If I have the antenna in, speeds are good and my RSSI is good. If, while it's connected, I remove it, within 2 seconds all the bars drop down to nothing (no LED lights), and my RSSI hits over -110, and I get shoved into NationalAccess. As soon as I put the tack back in, within seconds I'm back to 3-4 bars, lower RSSI, and BroadbandAccess.

Any tips? I have my eye on an external antenna for the UM150, but I can't get it for some time because I'm a poor bum who eats discarded bread crust for dinner. For the time being I can leave the antenna as is, but it's bugging me that it's stuck in this weird mode...

Thanks in advance for any help!

[edit]
Okay, strange. Everything is fine now, for some reason it just "decided to work". I'm not getting the same stats as before I did the antenna bit, but at least it's working at a decent level now. I seriously have no clue why it worked out like it did. Anyhow, I'd still like to hear some feedback on this topic.
[/edit]

n6gn · Posted: Wed May 28, 2008 6:58 pm Post subject:

The external antenna connector (or even the test port connector for manufacturing) is a switch. It is designed to disconnect the internal antenna and route the signal to the external coaxial cable.

I think what happened is, as careful as you thought you were, your thumbtack probably deformed the flexible arm of the switch and jammed it in the "I'm not touching anything when you remove the thumbtack" position.

As I infer you actually know, the external antenna input on most cards and devices is expecting a mating coaxial connecter rather than a thumbtack. This connector serves to make two connections to the coax, a center conductor and a shield. Speaker wires are not coaxial cable and even if you did manage to hook up to both the center and shield of the coaxial connector, they also are balanced rather than unbalanced transmission line. This means that in addition to the excess loss that their insulation probably has at UHF-microwave, the line itself would act like an antenna and radiate, if fed by an unbalanced source like your card.

I'd suggest that if you're going to homebrew an antenna that you get a correct coaxial adapter cable, study up on coaxial cable and connectors a little, and study up on antennas and how to mount and connect to various types a lot. It may not be quick but I'm sure in the end that it can give you better results.

n6gn

Derjyn · EVDO Newbie Joined: 24 May 2008 Posts: 7

While I'd definitely love to dive deeper into it, and make a project for a powerful antenna... it was really a spur of the moment thing. Ballsy too I admit, but I was lucky.

I actually assumed that was what I had done, gotten the switch inside stuck. I just wasn't sure if it was what really happened. I believe that's how I got it fixed, popping the switch back. I had put a finer pin and lightly pushed it in the connector until there was the slightest resistance, then pushed the pin to that side and pulled it back out. There was a slight click (more of a feel than a sound), but I can't be sure that it was the switch... but it's back to normal, so who knows?

Thanks for the insight n6gn. As with all hobbies- if and when I get into the antenna bit, I'll research it to death. I was just having a hard time finding anything directly related to the UM150's external antenna.

evdoRV · Posted: Sat May 31, 2008 7:47 am Post subject:

Just curious, but what did you use to calculate the antenna wavelength?

Transmitter's require a matching load for it to work properly. Likewise this load must be matched with the proper cable. The cable must match the impedance of the output of the transmitter and also the impedance of the antenna.

The wavelength of the antenna is based on the frequency of the transmitter, which is why antenna's are designed very specific to the device they are connected to.

If you did see a signal at all it's probably because you had a decent signal to begin with. But continous use of a mis-matched antenna will only cause problems in the transmitter portion of the UM150. At the very least the circuitry will shut itself down to help protect the transmitter from burning itself out.