144mhz NMO mounted antenna for 121.x?

Started by rroberts, January 17, 2010, 02:10:26 AM

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rroberts

I passed my Technician rating today for HAM 8)  I have to wait about two weeks and I will be getting my call sign.  While I wait for that I want to setup my car. 

I will be installing a ic-7000 (ICOM) and the antenna will be an NMO mounted antenna.  I'm going to have a pro shop install the NMO mount and run the coax.  I think I will actually have them install 2 NMO's, the second being a 70cm. 
The main reason that I went to get my technician was so that I could understand the radio gear a little better than the basics we typically teach in CAP.  I wanted to have a way that I could listen for 121.x signals so that we could tell when our ground team was getting in the general area.  Once there, we could then use the DF gear to track the location. 

Can a 144 / 220 / 440 do a good job at "hearing" both 121.x and the newer 406, or should I get a specialized antenna just for 121.x and or 406?


Thanks,
Bob Roberts

SarMaster

For Receive only you can use just about anything.... You should have no problem with a 2m antenna to receive 121.5 or 406.  Just remember 406 is only transmitted for a few mili seconds every 50 seconds or so.

The best way to DF with just a 'Scanner'  is to go off freq to act asn an attenuator....so the louder the signal gets the farther off freq you tune your receiver.   Its not uncommon to end up tunning to 135 or so as you get close to the signal.

I dont have the specs but does the IC-7000 do Airband receive?   if so you may need only one antenna...
Semper Gumby!

rroberts

That's true,  I keep forgetting that the antenna length is most important for the transmit and not the receive.  I haven't tried it, but I'm assuming the ELT won't respond to a CQ  ::)

SarMaster

hmmmm....  Well..that depends.  I wired up a PicCon  (www.byonics.com)  to a ELT trainer... So a DTMF sequence will turn it on and off...Yes i know its sneaky.

Kinda entertaining when the team is right on top of the trainer and the signal goes silent.
Semper Gumby!

sardak

Here is a CAP Thread entitled "Anyone found a good 121.5 mobile antenna?" http://captalk.net/index.php?topic=5110  I mention in the thread that from many years of experience using both, the antenna tuned to 121.5 always outperformed a 5/8 wave 2m antenna (which is a couple of feet longer than the aircraft antenna) for monitoring 121.5.

Thank you for wanting to monitor 121.5 while mobile. There are still too many CAP ground/UDF team members who don't understand the importance of that doing that. You can also DF the signal quite well, with practice, using just the single antenna.

Mike

JoeTomasone

#5
Quote from: sardak on January 17, 2010, 05:08:11 AM
Here is a CAP Thread entitled "Anyone found a good 121.5 mobile antenna?" http://captalk.net/index.php?topic=5110  I mention in the thread that from many years of experience using both, the antenna tuned to 121.5 always outperformed a 5/8 wave 2m antenna (which is a couple of feet longer than the aircraft antenna) for monitoring 121.5.

Thank you for wanting to monitor 121.5 while mobile. There are still too many CAP ground/UDF team members who don't understand the importance of that doing that. You can also DF the signal quite well, with practice, using just the single antenna.

Mike


Agreed, and I updated that thread with my findings.

One thing to keep in mind is that (terrain depending, of course) the range of a 121.5 signal isn't impressive, especially given the range of your average repeater.   In practice, in flat-as-a-board Florida, I have detected ELTs as far away as 3 miles and as close as 1/4 mile.    There are several factors here that determine range that I won't completely elaborate on, but the height (and suitability) of both antennas, the power output of the ELT (which degrades as the battery does), and the sensitivity of the receiver on 121.5 are all important.    So while monitoring 121.5 is always a good idea, don't get the impression that you'll pick up anything in your town/county that's going off.   

To get an idea of what I am referring to (since you are a new ham), try this: Find an active ham repeater that has an ongoing QSO.   While monitoring it on one receiver, turn the receiver on your roof-mounted antenna to the repeater's input frequency and see how much of that traffic you can hear.


I wouldn't recommend setting up anything specifically for 406 since the bursts are so short (500ms) and infrequent (1 per minute).   You could set up a scanner in your vehicle with a decent scanner antenna (if same is permitted in your State, it may be prohibited and there may be an exception for ham radio operators) and have it scanning both 121.5 and 406.   I do this in my vehicle even though the scanner antenna isn't perfect on either frequency.   The 121.5 antenna is reserved for the MK4 Sniffer.


SarDragon

#6
I've tracked an ELT from over 4 miles away, but it was over relatively flat terrain. I had my L-per hooked to a 121.5-tuned roof mount antenna. The ELT was on top of an AF C-130.

BTW, regarding the thread title, an antenna tuned for 144 millihertz is really, really long, like 1.04 million kilometers.  :)
Dave Bowles
Maj, CAP
AT1, USN Retired
50 Year Member
Mitchell Award (unnumbered)
C/WO, CAP, Ret

rroberts

Quote from: SarDragon on January 17, 2010, 06:10:30 AM
BTW, regarding the thread title, an antenna tuned for 144 millihertz is really, really long, like 1.04 million kilometers.  :)

That could hamper my fuel economy a little carrying that much wire behind me...  Will also make turning the car interesting 8)

I need to pay better attention to the little and big letters.