Need ideas for a receive-only 27Mhz Mobile antenna

[SD2007]

I need a decent RX only antenna for my 4Runner that doesn't make me look like trucker bob. I'd really like to avoid the 1/4 wave vertical whip. Would a horizontal dipole with some loading coils do the trick? Has anyone tried something like this?

Thanks...

 

[k9rzz]

Using a horizontal antenna when everyone else is vertical will give you a 20 db (3 - 4 S units) loss right off the bat. A shortened antenna will be even worse.

I guess it depends upon how far you'd like to hear. With my whip stored inside the car, I'm hard pressed to hear the truckers 1/4 mile away.

I have a commercial CB whip from Fleet Farm that looks like a cell phone antenna and it actually works pretty well !

It looks like this, but a mag mount.

 

[N4JNW]

Considering you're going to be RX only, grab or make you a splitter that runs inline with your AM/FM stereo antenna. One would be relatively easy to make.

Go to Advance or Auto Zone, and get a AM/FM antenna extension cable. Yes, they make them. It will have a plug on one end that looks like the plug that your antenna coax running into your radio has. On the other end, there will be a jack that looks like the back of your stereo.

Cut the dude in half, and crimp on some cable TV 75 ohm connectors. You can get these for about $0.25 at a tv shop. Then, while at the tv shop, get a 3 way splitter.

Ok, now, take you another piece of coax that you wish to plug into your reciever, and crimp another 75 ohm cable connector on it, and put a BNC or PL-259 or whatever on the other end to plug into your radio.

Hook everything up, into the splitter, and tuck away in your dash behind your stereo, with your *new* recieve only cable running out of the dash.

Now, you can recieve off of your AM/FM antenna. You have NO antennas outside to be noticed or stolen. Granted, you can only recieve, and by any means DO NOT transmit through this, because it will feed directly into your stereo. But with the 75 ohm tv connectors, you probably won't anyway.

You get the drift.

 

[SD2007]

Thanks for the ideas guys, I'll probably give them both a try.

 

[SD2007]

Using a horizontal antenna when everyone else is vertical will give you a 20 db (3 - 4 S units) loss right off the bat.

Is this true for only dipoles? When listening to a weak station with my RS center loaded whip (#20-006) there seems to be no difference in signal strength when holding the scanner vertically or horizontally.

It turns out the FM radio antenna in my Toyota is some kind of odd dual-plug diversity something or other and I couldn't get my scanner to receive squat with it. I can't find a through-glass antenna that works well for 27 Mhz. I may have to resort to a roof mounted beast or give up.

 

[k9rzz]

Is this true for only dipoles? When listening to a weak station with my RS center loaded whip (#20-006) there seems to be no difference in signal strength when holding the scanner vertically or horizontally.

It turns out the FM radio antenna in my Toyota is some kind of odd dual-plug diversity something or other and I couldn't get my scanner to receive squat with it. I can't find a through-glass antenna that works well for 27 Mhz. I may have to resort to a roof mounted beast or give up.

Well, perhaps the signals you're hearing are loud enough that you won't notice the difference between vertical and horizontal. Cool !

I don't know much about through glass CB antennas. My guess is that there isn't much signal transfer through that tiny little capacitor to feed the sig through the glass, plus the antenna is a compromise to start with.

Good Luck!

John K9RZZ

 

[N4JNW]

Have you ever thought about just getting a el-cheapo mag mt CB antenna and throwing it up there?

I've seen some mag mt. CB antennae that were pretty small actually. I used to have one that was a rubber duck on top of a magnet. I think I gave like 10 bucks for it. Transmit on CB, it sucked badly. But, it recieved fairly well when it was on a decent groundplane.

Wal-Mart has some mag mt. CB antennae that are only about a ft and a half tall, with a loading coil in the middle. Those also suck for TX, but if you're recieving only, who cares?

For 10 or 15 bucks, try one, and if it don't work, return it.

 

[policeMonitor]

antenna for my 4Runner that doesn't make me look like trucker bob.QUOTE]



i look like a trucker bob?

 

[N4JNW]

Trucker Bob... Hahaha....

 

[SD2007]

Have you ever thought about just getting a el-cheapo mag mt CB antenna and throwing it up there?

I've seen some mag mt. CB antennae that were pretty small actually. I used to have one that was a rubber duck on top of a magnet. I think I gave like 10 bucks for it. Transmit on CB, it sucked badly. But, it recieved fairly well when it was on a decent groundplane.

Wal-Mart has some mag mt. CB antennae that are only about a ft and a half tall, with a loading coil in the middle. Those also suck for TX, but if you're recieving only, who cares?

For 10 or 15 bucks, try one, and if it don't work, return it.

Actually I'm thinking about getting a 24 to 36 inch "mirror mount" CB antenna and mounting it with a custom bracket where the hood meets the windshield on the driver's side. A mag mount on the roof would probably perform better but I'd never get my 4Runner in the garage. Would putting the antenna at the corner of the ground plane (hood) give bad results?

 

[jonny290]

psst - you can pop that mag mount off and lie the antenna down flat when you go in the garage

To be perfectly honest, the CB band is S9+ noise and static all around, and propagation is an absolute minimum right now. Unless the stations you're monitoring are within 4 miles and/or running illegal power, you likely won't hear them no matter what you slap down.

Antenna performance will increase linearly as you get the mounting spot closer to the center of the vehicle, and as the antenna approaches 102" in length. That should be enough to pick out the perfect antenna for your needs - and if you buy one and it sucks, just return it or ebay for a $5 loss, chalk it up to learning experience.

 

[gmclam]

CB Antenna

If want you really want to hear is CB; then forget the fact you are not transmitting. Get a good base loaded or center loaded antenna. The load matches (tunes) the wavelength and allows the physical length of the antenna to be shorter.

 

[SD2007]

psst - you can pop that mag mount off and lie the antenna down flat when you go in the garage

I'm not willing to accept the inconvenience of getting out of my car and messing with the antenna every time I enter or leave the garage. Aside from that, I'm bound to forget at least a few times and plow my antenna over.

For $30 I picked up a 24" top-loaded antenna from RS that's supposedly tuned for CB ch. 19. I'll try this in a few places around the vehicle and find a good compromise between performance and mounting practicality.

 

[SD2007]

I finally got the antenna mounted and wired up. It works great for CB reception and does quite well on VHF. My only problem is that I'm picking up spark ignition and other misc. electric motor noise from my car. Would adding a high-pass filter help knock out the noise? I thought I read somewhere that electrical arcs generally radiate in the 1-2 MHz range. Is that true?

 

[gmclam]

CB & noise

Make sure the antenna is GROUNDED. Do you have spark arrestor plugs on your auto? If noise is coming in through your power, a low pass filter on the power will help. I would think a high pass filter in line with the antenna will not filter the noise, without also filtering what you want to hear.

 

[SD2007]

Make sure the antenna is GROUNDED. Do you have spark arrestor plugs on your auto? If noise is coming in through your power, a low pass filter on the power will help. I would think a high pass filter in line with the antenna will not filter the noise, without also filtering what you want to hear.

The antenna is grounded to the vehicle chassis at the mounting bracket. I checked ground continuity from the antenna base to the hood, frame, and battery. Coax shield at the scanner is < 1 ohm to ground. I'm using a BR330T with the AA batteries as the power source, so no noise is coming from 12V power. The vehicles spark plugs are resistor type, I don't know what spark arrestor plugs are.

 

[gmclam]

Noise

Your installation sounds correct. Keep in mind that you're dealing with noise that is AC (or RF). While the DC resistance of your antenna to ground is less than 1 ohm, what is important is the AC impedance to ground. As an example, some people will mount antennas on a fiberglass (or other non-conductive) part of a vehicle and then run a wire (or several wires) to the vehicle chassis. The length of that wire is going to act like an antenna. If your antenna is mounted on the metal of your auto, that's great.

The resistor type plugs are the noise supression plugs. Now I am wondering if the problem is with your auto, or radio system. In other words, if another vehicle was parked next to yours with a CB (or other AM radio) will it pick up the same noise that you are picking up? If so it is your vehicle emitting the noise. This type of thing can be elusive to determine the source of.

 

[prcguy]

Back in the 70&#8217;s when I worked in a CB shop we sold an antenna called the &#8220;Intenna&#8221;. It was a very small matching box with two variable caps that mounted (and grounded) on one side of any window in the vehicle and a wire that stretched across the window and grounded to the other side. It made the entire vehicle into a &#8220;slot antenna&#8221; and they worked surprisingly well. I still have a sample around here 30 yrs later and will post a pic if I can find it. Their popularity dwindled because everyone was using amplifiers in their cars and these antennas would not handle the power, plus they were a ***** to install and hide in newer cars.
prcguy