Garage Door Opener Range.

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http://en.wikipedia.org/wiki/Isotropic_radiator 

Whoosh whoosh whoosh. 

John Fields

Agreed. 0 dB losses and 0 dBm gains were mostly so I didn't have to reach across the desk for my calculator. I made no effort to be accurate or even realistic. I simply wanted to demonstrate the effect of putting a passive repeater in the middle of the RF path. I can change the numbers to more closely reflect reality, but the added losses due to the passive repeater will not change things very much.

A 200 meter outside path will make a passive repeater even less desirable. If I leave the inside path at 10 meters, which I think is realistic for across a large garage, and use 200 meters for the distance between the car and the garage the path loss will be:

Without passive repeater. 210 meter path at 433 MHz: the loss is 72 dB.

With 200 meters plus the passive repeater at 10 meters, the loss is:

71.2 dB + 45.2 = 116 dB With this arrangement, the RF signal level at the receiver will be

-126 dBm, which is well below the receiver operating level.

That's a difference of 44 between using a passive repeater and a direct shot. Throwing in realistic antenna gains will make things even worse.

Quiz: Starting with a 1/4 wave whip antenna over a large ground, how short can this antenna be cut before the gain drops substantially? The numbers in the filenames are the antenna lengths in wavelengths. For example, monopole_0_125 is 0.125 wavelengths or 1/8th wavelength. I only ran 3 applicable lengths, but the gain figures should offer a clue that as long as the feed impedances are reasonably well matched, you can probably cut an antenna down to below 1/20th wavelength, and still have a fair amount of gain: wavelengths gain(dbm) 0.250 5.19 0.125 4.85 0.050 4.75 The VSWR is atrocious, but the gain is still mostly there.

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Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

I do wish you would use a 433 MHz or 315 Mhz rubber ducky antenna. I'll make it easy. Put a pizza platter on Jim's roof and ceiling. Add a 5/8 wave antenna with a suitable matching transformer, and I'll call it about 5 dBi gain. At 10 meters from the garage, that will probably work. At 200 meters, no way.

Sure. Like I said. Add some gain at any of the antennas to make up for the 39 dB difference in path loss between the direct versus the passive repeater, and it will work just fine. However 2.5dB gain per antenna is not going to make up for 39 dB of loss. Granted, you can run the signal levels down to the minimum detectable signal level, and possibly make it work. However, don't use my figures for doing that. I picked 0dB losses and 0dB gain antennas simply to illustrate the large difference in path loss between the two schemes. If you want a more accurate calculation, I can grind out the numbers. I'll need some details from Jim Thomson, such as the height of the garage, the height of the garage door opener, and the size of the garage.

I think differently. Using my numbers, the only way to compensate for the difference in losses is with antenna gain. Well, maybe increase the transmit power or put an RF amp in front of the receiver.

True. However, that doesn't make much difference if the antenna is outside the shielded garage. I consider that a necessity, whether using a passive repeater or a direct shot.

No, it's not. The situations that work is where there is absolutely no signal via the direct path. The basement of an emergency center is one. Another is 4 floors down in an underground garage. Jim's shielded garage may be very lossy, but is still leaky enough that cancellation (nulls) between the incident and passive repeated signals will be a problem.

Only if you're lucky. The problems with such calculations as mine is that they define the best possible conditions, where everything behaves exactly as predicted. Reality tends to suggest that this is most often not the case. Losses are never better than predicted. Power output and rx sensitivity are never up to spec. Path losses always have some extra obstructions that raise the losses. And, I haven't even thrown in Fresnel zone diffraction. There are no optimists in the RF business and antenna problems tend to be very strange:

Believe me, I have loved passive repeaters since I discovered one hiking in the hills above Big Bear Lake in southern Calif. When we finally reached the top of the mountain, there was a huge billboard on top. No advertising or embellishments. Just a billboard. I discovered later that it was a passive reflector for the microwave links used by AT&T to cross the country. I thought it was cool. Somewhat later, I blundered into periscope antennas, where a 45 degree reflector in the shape of an ellipse was perched on top of a tower, with a dish antenna pointing straight up at the reflector. Again I thought it was cool.

Then, I discovered that ecology and fiber optics had conspired to remove the billboards from the mountain tops. The FCC also took a dim view of the signal splatter produced by periscope antennas and effectively banned them. So much for my first love in antennas.

At various times in my checkered career, I've toyed with various forms of passive repeaters, and found little in the way of success. The math shows why, but I was more than willing to ignore the calculations in the hope that they might be wrong. When Wi-Fi took hold, I immediately resurrected the idea in a 200ft fir tree, pointing one dish at the WISP (wireless ISP) and the other down to my house. As long as I didn't mind climbing the tree to realign the antenna every few weeks, it worked fairly well.

I never make the same mistake twice. 5 or 10 times is more my style.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

What is the software model you've used here?

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Mike Perkins 
Video Solutions Ltd 
www.videosolutions.ltd.uk

Gentlemen, gentlemen! Really! The best forum for questions like this is uk.d-i-y (anything concerned with house building and home improvement in fact). I've added it. Let's hope for some positive contributions...

Throw the radio away - it has a huge security hole. Here's why. Remember the night stalker? He would just sneak into the house when the garage door was up and the driver's attention on driving in. Big security hole

What? I've often wondered how a lot of radio controlled devices work when they seem to have aerials concealed in their cases. I was always taught that they should be resonant at the frequency needed and putting hands and buildeings close by detunes them even before you think about the screening effects. How do Mobile phones ever work, one wonders.. grin. Brian

4NEC2: 4NEC2 does not have an easy way to produce presentable web pages, so I screen grabbed the JPG's using Irfanview and assembled the web page with jAlbum.

I also use EZNEC:

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Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

Do you believe the numbers it is giving you? 5.19 dB for a 1/4 wave ground plane antenna? Also, gain is in dB, not dBm. Just a little nit.

The often metal case has a plastic window to let the RF in and out. The windows needs to be only slightly larger than the antenna in order to function. This arrangement also has the advantage in cell phones at directing the RF away from the users head, thus reducing the SAR (specific absorption rate).

Antennas do NOT need to be resonant at the operating frequency. Certainly it helps, but it's not required. For example, the metal frame around the Apple iPhone 4 and iPhone 5, which acts as two antennas, is not resonant.

The users hand will detune any antenna in close proximity or by touching the antenna. I ran a crude test of that when it became an issue with the iPhone 4. The "difference" column shows the decrease in signal strength between a two finger grip and a stranglehold. While there is a substantial decrease in signal (6-12dB) for most phones, it's not catastrophic, except on the iPhone 4 (20-25dB).

Because RF is magic, but more likely, because users have learned to tolerate dropouts, dead zones, garbled audio, marginal hardware, limited quality, overpriced plans, billing errors, etc.

--
Jeff Liebermann     jeffl@cruzio.com 
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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Nice. :-) 

Thanks, 

John Fields

Cursitor Doom brought next idea :

The only thing you are allowed to improve, is the receiver. Have you looked at the receiver's (in the garage) antenna?

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Regards, 
        Harry (M1BYT) (L) 
http://www.ukradioamateur.co.uk

It is not that far away, assuming perfect ground, which explains the lower lobe to reflected upwards, which would explain 3 dB of gain. The remaining 2.19 dB sounds much like the dipole gain over an isotropic radiator. So actually, we should talk about 5.19 dBi gain or actually _directivity_. For electrically small antennas, the efficiency can be much less than 100 %, thus the _gain_ would be less.

When the physical dimensions of an antenna are reduced, the antenna effective capture area and hence the obtainable receiver power drops much slower. Of course, there must be a proper match between the (reactive) antenna and the receiver.

Sorry about the dB and dBm mixup. My sloppiness. Thanks.

Yes, I believe the gain figures. However, it doesn't matter because my point was that the CHANGE in gain with reduced antenna length is minimal. I've seen that in bench and field tests, so I know that works. The catch is that the antenna has to be impedance matched, which becomes increasingly more difficult and lossy as the antenna becomes shorter.

In general, unless I hit one of the limitations of NEC2, the patterns and gains of the models are fairly accurate. For this monopole, the added gain comes from reflections off the ground plane. The situation is similar to a mobile HF antenna, where the length of the antenna is considerably shorter than 1/4 wavelength. As long as the antenna is reasonably well matched and the matching losses are included in the calculations, then a short antenna can work almost as well as a real

1/4 antenna.

However, reality is never as good as the models predict. In this case, I'm using an infinite perfect ground. I have yet to find one of those outside of an antenna range. The matching losses for the shortened antennas will add more losses. At best, the 5.19dB gain will be a best case maximum, with reality being somewhat less.

You can sorta see the effects of height above ground in: The animation was originally intended to show the effect of antenna height on the pattern, but it also shows the changes in gain. Closest to the ground, it shows 5.24 dB gain. Highest shows 7.08 dB. Both are much larger than the traditional 2.18 dB gain of a free space dipole.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

Cursitor Doom scribbled...

As garage door openers are used to set off IEDs, I see no reason for anyone to be giving info on how to increase their range.

Good explanation and you're correct about the directivity. Having gain in the wrong direction isn't very useful. In the case of the shortened monopole, the peak gain is in roughly the correct direction needed to be useful, so it's not an issue. However, other antenna configurations can cause problems. For example, here's an animated GIF of a common discone antenna. Up to about 400 MHz, the major lobe (i.e. maximum gain) is roughly horizontal, making the antenna quite useable. However, between 400 and 1000 MHz, most of the RF goes straight up. There's little RF left at the horizon, where it's needed. Such an antenna might be good for listening to airplanes, but not terrestrial stations.

The radiation efficiency and internal losses are included in the gain calculation. For example, if I made the antenna from lossy material, it would show up as a loss in gain. However, for a fairly close to ideal antenna, the radiation efficiency barely changes.

wavelengths gain Efficiency Radiation (db) Efficiency 0.250 5.19 100% 99.93% 0.125 4.85 100% 99.66% 0.050 4.75 100% 99.09%

Again, the problem is matching the impedance of the shortened antenna. The losses are not in the antenna. They're in the matching circuit.

wavelengths gain(db) VSWR (50ohms) 0.250 5.19 1.74:1 0.125 4.85 158:1 0.050 4.75 5954:1

The 0.050 wavelength monopole shows a feed impedance of 1.52-j707 which is going to be verrrrrry difficult to match efficiently to 50 ohms. Got a 500:1 transformer handy?

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

Suppose I simply made a dipole half inside and half outside? Would that improve my reception? ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

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As far as I know, the only real way to increase the range of IEDs is 
to make them bigger or to arrange for them to explode above ground. 

As far as their triggers go, I believe the device of choice for 
serious bombers is a cell phone. 

John Fields