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direction finding
As far as VHF, I know a couple here in town I've seen using arrays
for direction finding. But they used four verticals in a square. It worked quite well. We had one guy that kept jamming one of the repeaters, and the user of said array was easily capable of tracking him down to a shopping center parking lot where the guy was sitting in his car. I suspect the expression on the jammers face when the tracker tapped on his window was priceless. :) The device in the car used an array of LED's in a cross configuration if I remember right. He had the antenna array on the back of his truck. I don't recall the element spacing. Likely a Doppler direction-finder, possibly a Ramsey DDF-1 or one of the others of that sort. They aren't perfect... definitely subject to errors due to multipath, and they works best when looking for a relatively clean, non-modulated carrier. Fun, though... I've used mine on several occasions to locate sources of QRM or jamming. I've seen similar 4-vertical arrays on the roofs of some police cars in my area. I infer that they're used to track down cars which have had their Lojack locators triggered, after a car theft. -- Dave Platt AE6EO Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
direction finding
On 8/18/2011 5:49 PM, Dave Platt wrote:
I've seen similar 4-vertical arrays on the roofs of some police cars in my area. I infer that they're used to track down cars which have had their Lojack locators triggered, after a car theft. You are correct with respect to LoJack, which radiates in the high VHF band. They do pseudo-doppler direction finding. I don't know how fashionable LoJack is these days... the sophisticated thieves know how to jam it. The widespread deployment of in-car GPS/cell (On-Star, etc) covers a lot of the other cases. Face it, most car thieves aren't all that smart. |
direction finding
"Jeff Liebermann" wrote in message ... snip Mini-rant: Why do almost ALL commerical radios have tx timeout timers, while almost ALL ham radios don't have this feature? Is it because we're all known to be gasbags? Timeout? Why the very *impudence* of that radio!!! All kidding aside, we had a major repeater outage in San Diego some years ago. A 50w mobile, in the middle of a hotel parking lot, was on the air for hours, holdng the repeater. The T-hunters who found the car ID'ed the owner by his callsign plate and contacted him in the hotel. It was the old microphone-between-the-seat-cushions trick. Timeout would have helped that day. "Sal" |
direction finding
In article ,
Jeff Liebermann wrote: Fun, though... I've used mine on several occasions to locate sources of QRM or jamming. Ummm.... How did you get it to produce a usable bearing if there were two transmitters on the air at the same time? I can see if it you waited until the jammer was the only carrier, but few jammers allow that to happen. Mostly we use doppler for the traditional stuck transmitter. A lot of the jamming in question is jamming of repeaters, so you are DF'ing on the repeater's input frequency. If you have a rough idea where the jammer is operating, you can often get a Doppler detector (e.g. on a car) into that area and get a decent signal from it when he's jamming... but be far enough away from the person being jammed that you don't hear any of his signal (although the repeater does). A Dopper rig on the roof of a car has a far smaller line-of-sight radius than a repeater receiver mounted up on a high building or hilltop. -- Dave Platt AE6EO Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
direction finding
On Fri, 19 Aug 2011 12:59:57 -0700, "Sal" wrote:
"Jeff Liebermann" wrote in message .. . snip Mini-rant: Why do almost ALL commerical radios have tx timeout timers, while almost ALL ham radios don't have this feature? Is it because we're all known to be gasbags? Yes. That's also why many Japanese ham radios are severely lacking in microphone gain. I guess they assume that all Americans are always yelling. Timeout? Why the very *impudence* of that radio!!! Ok, bad choice of a name for the timer. Perhaps "anti-ratchet mouth" or "monolog restrictor" might be better. All kidding aside, we had a major repeater outage in San Diego some years ago. A 50w mobile, in the middle of a hotel parking lot, was on the air for hours, holdng the repeater. The T-hunters who found the car ID'ed the owner by his callsign plate and contacted him in the hotel. It was the old microphone-between-the-seat-cushions trick. Timeout would have helped that day. Yep. It's all too common. My guess is one every month or so. Unfortunately, I managed to jam the microphone under the debris pile that usually clutters the passenger seat, jamming the local repeater. I was easily identified by the classical music in the background since nobody else around listens to classical music. What's really irritating are the number of packet radios that end up on repeater frequencies. Usually, we can decode the call sign and contact the culprit. Occasionally, there's no call sign as the owner is trying to setup his TNC. There's also the stuck transmitter on commercial and public safety frequencies. Those are fun and gets us plenty of points from the various agencies. Extra credit to the local comm shop that likes to defeat the timeout timer, for no obvious reason. My favorite stuck transmitter was on marine channel 16. There was a very strong carrier on Ch 16, that could be heard all over Monterey Bay. Everyone dug out their DF hardware and the searching began. Strong signal, no modulation, 100% duty cycle. This should be easy. Right... After 5 days of bad guesses and dead ends, the culprit turned out to be a marine base station on Fremont Pk (3,000ft). It was rarely used and was only functional in order to maintain the license. Something went wrong with the DC wireline control system (with no timer) which stuck it on the air. Why so difficult? The strong signal created a wide assortment of reflections. Everyone had trouble separating the strong reflections from the incident signal with the Doppler DF boxes. I was using my rotating antenna kludge, but stupidly used a 5 element yagi tuned to 146Mhz. At 156.8Mhz, I later discovered that it had almost as much gain towards the back of the antenna as it did in the forward direction. Incidentally, there's another difference between ham and commercial mobiles. Commercial mobiles have a PL hang up function, where the receiver is in PL decode when the mic is hung in the hanger, and goes to carrier squelch when removed. Many ham mobiles seem to be missing this useful feature. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
direction finding
Jeff Liebermann wrote:
Incidentally, there's another difference between ham and commercial mobiles. Commercial mobiles have a PL hang up function, where the receiver is in PL decode when the mic is hung in the hanger, and goes to carrier squelch when removed. Many ham mobiles seem to be missing this useful feature. This is so the commercial operator listen to the whole channel (and not just his company's CTCSS filtered traffic) prior to transmitting. A typical ham channel is continuously monitored and everyone has the same CTCSS tone. |
direction finding
On 19 Aug 2011 21:00:51 GMT, dave wrote:
Jeff Liebermann wrote: Incidentally, there's another difference between ham and commercial mobiles. Commercial mobiles have a PL hang up function, where the receiver is in PL decode when the mic is hung in the hanger, and goes to carrier squelch when removed. Many ham mobiles seem to be missing this useful feature. This is so the commercial operator listen to the whole channel (and not just his company's CTCSS filtered traffic) prior to transmitting. A typical ham channel is continuously monitored and everyone has the same CTCSS tone. Sure, but we have several other distant repeaters on the same channels I like to monitor. I also hear plenty of RF leaks and odd sources of RF as I drive around. So, I turn on the PL decode. With a ham radio, I just pickup the mic, the receiver stays in PL decode, and proceed to talk. With the Maxtrac I currently use in the car, as soon as I pickup the mic, I hear all the junk, which helps to make sure I'm not causing a problem by talking over other users on the distant repeaters. (We have somewhat overlapping coverage areas). Also, when in PL decode, there's about a 600msec delay before the rx squelch opens, which obliterates the first two syllables. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
direction finding
Jeff Liebermann wrote:
TNC. There's also the stuck transmitter on commercial and public safety frequencies. Those are fun and gets us plenty of points from the various agencies. Extra credit to the local comm shop that likes to defeat the timeout timer, for no obvious reason. It does not seem the public safety radio devices always had this timeout timer either. Before the public safety services here switched to a digital system, they used NBFM equipment from companies like Motorola, and it was very common to hear stuck transmitters that blocked an entire repeater channel. (which often consists of multiple receivers and transmitters over a city area) Police used mobile transceivers in the cars on VHF and portables on UHF so it usually was possible to alert the users on the other channel and hope the stuck PTT key was found that way, but on major events like queen's day it often happened that the portables channel was jammed for half an hour or so before the culprit was found. Of course the equipment was aging by that time, it probably was from the eighties. Now they are all on TETRA and AFAIK that system allows the operator to just tune out stuck transmitters or devices that fell into wrong hands. |
direction finding
On 20 Aug 2011 08:26:15 GMT, Rob wrote:
Jeff Liebermann wrote: TNC. There's also the stuck transmitter on commercial and public safety frequencies. Those are fun and gets us plenty of points from the various agencies. Extra credit to the local comm shop that likes to defeat the timeout timer, for no obvious reason. It does not seem the public safety radio devices always had this timeout timer either. Methinks that just about every radio, by Motorola and Kenwood have a transmit timeout timer. http://802.11junk.com/jeffl/pics/drivel/slides/too-many-radios.html The dip switch programmable radios (SP10) do not have a timer, but literally every radio that requires a computah to setup has a timer. Note that I was talking about mobile radios all of which are programmable, not handhelds. Before the public safety services here switched to a digital system, they used NBFM equipment from companies like Motorola, and it was very common to hear stuck transmitters that blocked an entire repeater channel. (which often consists of multiple receivers and transmitters over a city area) That's a programming issue. The timer is off by default and must be enabled and set for a specific number of seconds. 60 seconds is the typical timeout. Digital radio comes in many flavors. One of the big advantages of digital is that the repeater and channel can handle more than one transmission at a time (either by TDM or FDM). Therefore a stuck digital transmitter does not timeout or disable the repeater. In addition, many such radios are full duplex, which allows the dispatcher or comm shop to remotely disable the transmitter and yell at the user. Whether the PSAP is setup and trained to do this is debatable. Police used mobile transceivers in the cars on VHF and portables on UHF so it usually was possible to alert the users on the other channel and hope the stuck PTT key was found that way, but on major events like queen's day it often happened that the portables channel was jammed for half an hour or so before the culprit was found. Yep. Simply identifying the stuck transmitter is often insufficient. I've heard stuck transmitters in vehicles rolling down the freeway for an hour with a stuck transmitter. It was obvious who it was but nothing short of an RF seeking missile was going to get their attention. In one case, someone noticed that they were listening to specific local radio station. The dispatcher called the radio station and had them make a special announcement to the driver to unstick the transmitter. It worked, instantly. Of course the equipment was aging by that time, it probably was from the eighties. Now they are all on TETRA and AFAIK that system allows the operator to just tune out stuck transmitters or devices that fell into wrong hands. Yep. Nice feature. In the US, the high end radios are P25. Various other narrow band solutions exist, most of which have remote programming and remote deactivation. Despite all the government programs intended to provide interoperability, the manufacturers (and the FCC) have gone out of their way to introduce proprietary enhancements and introduce intentional incompatibility. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
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