Recommend an inexpensive mobile antenna for 45mhz?

[TURNz]

I have a Bearcat 350 scanner in my truck with the suction cup antenna. Everything is temporarily rigged up. I am going to be swapping it over to my work vehicle and would like to get a descent antenna to get better range. The channels I need are all in 45mhz range. I have seen some antennas at Radio Shack, but I don't know what is good. There is also a Bearcat dealer I guess near me. I'd like to at least have an idea what I am talking about if I go there or shop online.

[goofy]

who uses 45mhz these days?

Back in the day, crappy old cordless phones used to use 48MHZ.

[goofy]

There are two things you could do if you can't find purpose built equipment. You could use a 6m ham antenna, slightly lengthened. Or you could use a cb antenna with part of the coil shorted...

An antenna analyzer would be a nice to have for this kind of thing.

[TURNz]

Local police are on 45.1 and 3 other channels that start with 45.

[Night Crawler]

Get a 102" whip and cut it to 62 1/2"

[TURNz]

Get a 102" whip and cut it to 62 1/2"

So you are saying to use a 102" CB antenna like this?
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I guess I would just need an adaptor then.

[Night Crawler]

That's all you need cut it down and you'll have a 1/4 antenna for 45mhz
The formula to figure out the length is 234/frequency so 234/45.1mhz = 62 1/2 inches.

[linx]

Get a 102" whip and cut it to 62 1/2"

Bingo! Night Crawler answered with the EXACT same thing I was going to answer with.

Can't get any better/cheaper than that!

[TURNz]

So is there an adaptor to change the CB hook-up to the BNC looking hook up on my scanner?

I'm hoping to finally get around to doing this over the weekend.

['Doc]

Yes, there are adaptors for that, and the same place ought'a have them.
- 'Doc

(Not a biggy at all, but it's 62 1/4". )

[TURNz]

It looks like Radio Shack (online) has the 102" for $20, 20ft of coax for $16, and an adaptor to connect an external antenna to a Cobra Portable CB, which looks like a CB to BNC connector for $8. I'll be going to the RS after work and see how much of it they carry in stock.

I'll make the mount from angle iron. I think that's all I need.

[TURNz]

Local Shack no longer carries anything CB related except coax. Stopped at a CB shop and he wants $40 for a steel 102". Could his be anything better than what Radio Shack has?

[hilltop 439]

I doute it probably the same thing just double the price all it is is a stainless steel taperd whip can't get any more fancy than that. 45MHZ that is a funny frequency didn't know anyone even used it . That's right next to the 6 meter Ham band

['Doc]

Considering that you'd have to cut the thing off no matter who made it, I think just about any 102 inch whip would work for you. You might ask around, someone may have a used one for sale. Or MFJ sells a 108 inch whip, no idea what their price is though.
- 'Doc

[TURNz]

Ok, I've got my whip, stud kit, coax, bnc adaptor.

I have a couple questions.
1. I read somewhere to use a fine tooth saw to cut, does this really matter? I was thinking dremel tool or angle grinder and a file.
2. My k40 antenna was cut at an angle. Is there a reason for that? Should I cut at an angle to make a point?
3. I was thinking that locktite might be necesary due to vehicle vibration, but will it hurt antenna efficiency?
4. Should I protect the end of the coax from rain?

This is for my scanner, and I know its less critical, but I will eventually have a two way mobile to transmit on the same freq. Will another antenna set up the same way be good for that as well?

['Doc]

1. I read somewhere to use a fine tooth saw to cut, does this really matter? I was thinking dremel tool or angle grinder and a file.

That whip is very tough, so think large when you're talking about a cutting tool. A dremel will certainly do it, but have more than just one or two cutting wheels, you know?

2. My k40 antenna was cut at an angle. Is there a reason for that? Should I cut at an angle to make a point?

Probably just didn't cut the thing 'square'. A sharp point isn't the best idea in the world, more susceptible to noise/static/etc.

3. I was thinking that locktite might be necesary due to vehicle vibration, but will it hurt antenna efficiency?

It shouldn't be a problem, and certainly helps keeping things together.

4. Should I protect the end of the coax from rain?

Not a bad idea, but also not one that I'd waste a lot of good worry on unnecessarily. Got some silicon sealant?

This is for my scanner, and I know its less critical, but I will eventually have a two way mobile to transmit on the same freq. Will another antenna set up the same way be good for that as well?

Transmitting antennas are more critical than receiving antennas. Would another kind/style be more better? No idea, but there will be some changes that should be made. The length of that antenna just isn't very critical for receiving only, but it certainly is for use as a transmitting antenna. The difference between being in the 'ball-park' and getting on base, sort of.
- 'Doc

[TURNz]

I understand that the length is more critical with TX. But isn't the length of 62.25" exact enough, assuming I can measure and cut to exactly that? Also, where is that length measured from?

At first I was thinking from the bottom of the threads on the antenna. But now that I look at the stud kit, I'm thinking it needs to be measured from the bracket, thus being exact length from the grounding point. Then again, the antenna is isolated from ground, so perhaps the measurement should be taken from the point that the coax attaches.

I realize that we are talking about differences of less than an inch, but which is correct?

[TURNz]

I understand that the length is more critical with TX. But isn't the length of 62.25" exact enough, assuming I can measure and cut to exactly that? Also, where is that length measured from?

At first I was thinking from the bottom of the threads on the antenna. But now that I look at the stud kit, I'm thinking it needs to be measured from the bracket, thus being exact length from the grounding point. Then again, the antenna is isolated from ground, so perhaps the measurement should be taken from the point that the coax attaches.

I realize that we are talking about differences of less than an inch, but which is correct?

Bump for these last couple questions.

['Doc]

How do you come up with that 62.5 inch length? First, that's a 1/4 wave length. An easy way to figure that is by dividing 234 by the frequency in Mhz, (234 / 45.1Mhz = 5.18 feet or about 62.25 inches). Not exactly the same numbers you got, but close enough.
So where do you measure that 62=something inches to - from? The antenna part starts where the feed line ends. That usually means you measure form the tip of the thing to the base of the antenna mount, cuz that's where the feed line ends. The mount is actually part of the antenna so should be counted in it's length. There are exceptions to that. One of those exceptions usually involve something like an impedance matching 'part' of the antenna. I doubt if that's going to be a concern for your antenna though.
Length isn't all that 'critical' for receiving antennas. It could make -some- difference, of course, but that difference isn't going to be very much at all.
Any of that help?
- 'Doc

[TURNz]

I fabbed up my bracket last week. I have it mounted to the truck, but not properly grounded yet. The bracket that I took the idea from was mounted with two screws through it into the sheet metal of the truck. That doesn't seem like a good ground to me. I was thinking of running a stud on the bracket and running a wire or braid to graound. I don't see anywhere less than 3 ft away though to use for a ground.

I am going with 62.25". I'll measure from the bracket, since thats where the stud kit is grounded at.

When I mocked up the bracket, I added the stud kit and antenna at full 102" length. It's humungous.

[crazy_cooter]

I doute it probably the same thing just double the price all it is is a stainless steel taperd whip can't get any more fancy than that. 45MHZ that is a funny frequency didn't know anyone even used it . That's right next to the 6 meter Ham band

The Ohio State Highway Patrol used to be on 45 MHz until they switched to that new system..I dont know the proper term for it. It's like 800 MHz I think

[Night Crawler]

Trunking System

['Doc]

That 'sheet metal' makes a dandy "ground" all by it's self. If you just want to run another ground wire, have at it, but it's not necessary.
- 'Doc

[drdx]

Man, you guys are working to hard on this. He's just going to listen, so why not just Y into the FM antenna on the truck and see how well it works? He could even add some loading inline with the feed to enhance it but I bet that the radio antenna would work fine. Or, a short loaded scanner antenna to avoid the really big antenna for just listening.

By the way, a wilson replacement whip is 63 inches or so, for the trunk version, if you can source one of those. That and a $5 ferrule deal and you're all set.

-drdx

[TURNz]

I am only receiving at this time, but I will eventually be transmitting on the same frequency and I was hoping to duplicate this set up for that.