antenna length for video xmtr

Greetings Folks, I have a video camera/transmitter unit that transmits in the 900 MHz to 1200 MHz range according to the spec sheet. It currently has a wire for the antenna that's 3.5 inches long. For optimum transmitting doesn't the antenna length need to be a length that divides evenly into wavelength? I will be enclosing the camera in a small plastic waterproof enclosure with batteries and a light. Then it will be lowered 70 feet down a 6 inch diameter iron pipe. It will be used to inspect the well casing and pump as well as to watch the water as it is being pumped. I want the best signal possible and was wondering how long to make the antenna if the present one doesn't work. Right now, in open air, a good signal is present at 100 feet. But I don't know how well it will work when lowered down the iron pipe. Thank You, Eric R Snow

Reply to
Eric R Snow
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That's what I'm afraid of. But it's much easier to keep the camera dry and to maneuver it around if it's only connected by a rope. So I'm gonna try it first. ERS

Reply to
Eric R Snow

Yeah Bob, I tried the cable as rope thing and the cable wrapped around the rope and had problems with manipulation and strength problems. I guess I'll lower tthe camera down and if there are problems then go to the extra trouble to make a case with sealed leads coming through. . ERS

Reply to
Eric R Snow

If you're going to be lowering the thing down an iron pipe, I would think you would do FAR better simply running the signal over a cable than you ever will with a wireless link.

Bob M.

Reply to
Bob Myers

There's a problem with the "rope" being a coaxial cable, or at least having a mini-coax wrapped around it?

You're going to have REAL problems with an RF link in this case, trust me...

Bob M.

Reply to
Bob Myers

You need to run your video and power over a cable (easy to do using 4 conductor telephone wire). The below cams would be easy and cheap to use with a little tweeking/modding.

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Reply to
Si Ballenger

1/4 wavelength is the best length. L=C/F (C=3E8) For 900 MHz this is 333 cm. Cut the antenna a bit longer and trim until you get best signal strength. If it will fit, silver plated narrow gauge welding rod works well.

Actually, with the transmitter down a metal pipe you could get a wave guide effect, in which case the signal will boom out.

Cap

Reply to
Captain

^^ ?

No, 3e8/9e8 = 1/3m. Thats the wavelength in meters.

1/4 of that is 8.33cm, or 3.28 in. Your antenna is already close to 1/4 of a wavelength.

Regards, Bob Monsen

Reply to
Robert Monsen

Greetyings Bob, It's interesting that the calculations comes so close to the length. Since the thing transmits anywhere in the 900 to 1200 range the antenna being 3.5 inches long would seem to be covering that range. If the antenna needs to be extended and a shielded wire is used until the last 3.5 inches of wire will the effect be the same as the antenna being connected directly to the circuit board as it is now? Thanks, eric

Reply to
Eric R Snow

On Friday 17 September 2004 04:53 pm, Eric R Snow did deign to grace us with the following:

What strength problems? If the rope will hold the camera without the cable, what makes the rope becomes weaker when there's a cable dangling alongside it? You leave some slack at the camera end, and let the cable support only its own weight. You could keep them at the same length with some kind of capstan.

Good Luck! Rich

Reply to
Rich Grise

Rich, The video cable by itself was too weak. Wrapped arond the rope it was a mess. ERS

Reply to
Eric R Snow

333

guide

Mea culpa! I thought a foot was a bit long for the frequency! Cap

Reply to
Captain

The camera is small. The casing with batteries and LED lights is the big part. ERS

Reply to
Eric R Snow

Cable TV companies use a cable consisting of a coax and a stronger support wire for the drops from a utility pole to a house - perhaps you could use that... (Belden 7983A, for example)

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Peter Bennett, VE7CEI  
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Reply to
Peter Bennett

How much weight are you trying to drop down your well? Most cams small enough to drop down the well casing with the pump still installed weigh only a couple of ounces at most. Bullet types of cams usually have a threaded fitting on the rear in which a ~1' length piece of threaded rod can be attached. Tape the cam cable to the threaded rod so all hangs vertically. You can see the threaded hole on the rear of the little IR cam on the below page (this cam is small, but the bullet cams are even thinner). The cable with this cam probably could withstand a 50lb pull if it got stuck. The rear of the cam would need to be tapered to the threaded rod so the edge wouldn't catch on a casing misalignment and such. Sounds like you are trying to put too large a cam setup down your casing.

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Reply to
Si Ballenger

I've only pulled a couple of well pumps, but I wouldn't be putting all that stuff down the well casing. You are just asking for trouble. Go to google search to see what the well inspectors use. An IR bullet cam on the end of cheap cat5 cable will be plenty strong enough to support the cam and pull back out if it gets in a bind. Below is a cam you might want to look into. YMMV!

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Reply to
Si Ballenger

On Sunday 19 September 2004 07:42 am, Eric R Snow did deign to grace us with the following:

What part of "alongside" don't you understand?

Thanks, Rich

Reply to
Rich Grise

On Sunday 19 September 2004 12:04 pm, Si Ballenger did deign to grace us with the following:

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And leave the batteries topside, of course.

Cheers! Rich

Reply to
Rich Grise

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