Undercover antenna

[califzeph]

So, here is my idea:

I want to be out on foot with my Pro-97 pocket scanner, without a big 'ol rubber duckie sticking out. So, I thought I might try taking a piece of coax, with a BNC connector on the end, maybe 2 or 3 feet long, and attach it to the inside of a jacket, like up the back and down an arm of the jacket, or similar. Do you all think this would make a decent antenna? I mostly monitor a trunked system at 867-868 Mhz. One of the systems I monitor comes in pretty well with no antenna!
Any thoughts on what would be a good length of cable to use? Should I stripoff the outer black layer of rubber? How about the layer of shielding?
How does having the antenna that close to my body affect it's performance?
Or am I just nuts?

 

[N_Jay]

Do you all think this would make a decent antenna?

Coax is transmission line. What are you going to use an an antenna?

Any thoughts on what would be a good length of cable to use?

Enough to get to where you want the antenna to be.

Should I stripoff the outer black layer of rubber? How about the layer of shielding?

If you want it to be an antenna you need to take the shielding off. Or better yet fold it back for a coaxial dipole.

Taking the sheilding off with the black rubber layer in is REAL hard to do!

How does having the antenna that close to my body affect it's performance?

In general it degrades the signal. You body is conductive so it "shields" the antenna. In some cases it may seem to help, as anything in the near field also becomes part of the antenna.

Or am I just nuts?

Most likely! :twisted:

 

[bsavery]

If you receive the 800 Mhz system fairly well without an antenna, then try it with a stubbie like the Diamond RH3.

Bob

 

[califzeph]

Well, I got the idea from seeing an "in-car" scanner antenna on some website. It looked like a peice of coax with the sheilding stripped away and stuck to the windshield with suction cups.
I only get one trunked system without an antenna, Roseville PD. I must be very close to their repeater. The other one I "frequent", Sacramento County Sheriff, I need a pretty decent antenna for.

 

[RolnCode3]

Well, I got the idea from seeing an "in-car" scanner antenna on some website. It looked like a peice of coax with the sheilding stripped away and stuck to the windshield with suction cups.
I only get one trunked system without an antenna, Roseville PD. I must be very close to their repeater. The other one I "frequent", Sacramento County Sheriff, I need a pretty decent antenna for.

You're not alone. The Sac County TRS has been the bane of me since I got into this hobby. I have by FAR the most trouble picking up this system. You will need a decent antenna, and even then it can be hit and miss.

 

[Altpensacola]

No, I'm not a scanner nut.

and attach it to the inside of a jacket, like up the back and down an arm of the jacket, or similar.

No sir I don't have a question, I just happen to have my hand raised....

 

[kb2vxa]

Hi guys,

Oh good grief, where has all the logic gone? You're looking at the wrong end of the portable, think earphone. The whole pocket portable can be hidden in an inside pocket, right? Now you want to shut the darn thing up so the cop you're standing next to won't arrest you for carrying an illegal receiver. (Laws go back to the 1930s and deal with HF) The simple solution is headphones, he'll think you're listening to an iPod.

 

[kb5udf]

Actually It's not such a crazy idea

You might check http://www.qsl.net/n9zia/wireless/col432/
for information on how to make coaxial collinear antennas. The above
link sums it up better than I could. I have made these types of antennas
but for being suspended vertically, not coiled about clothing. By coiling
around an antenna, you could be grossly distoring the pattern, thereby
negating the effect of any gain you might have, in practice. If you want
to make one for reception in the 860 mhz region, just cut the dimensions/lengths in half from the 432 amateur band antenna mentioned in the article, and you don't have to worry much about the ground plane section, as other articles mention this is only critical for SWR match on transmit.

However, I concur with other writers that commercial 800 antenna would probably be the way to go.

KB5UDF

 

[Al42]

Re: Actually It's not such a crazy idea

You might check http://www.qsl.net/n9zia/wireless/col432/
for information on how to make coaxial collinear antennas.

Or just a coaxial antenna. Cut 1/4 wavelength (at the frequency you want to monitor - about 3.2 inches for 800 MHz) plus about 2 inches of outer insulation off a piece of coax. Fold the shield back on itself, and then continue until it's folded back on the outer insulation. Trim the shield and center conductor so that they're each 1/4 wavelength. Put a connector on the other end of the coax.

The antenna is actually a 1/2 wavelength dipole. Hang it inside your jacket or shirt or drop it down your pant leg.

 

[kb2vxa]

Hi guys,

"...how to make coaxial collinear antennas."

OK, let's clear up another misconception, ain't no such thing as a "coaxial colinear" antenna, it's an oxymoron. There is the colinear which is multiple half waves in phase and there is the vertical dipole, otherwise known as the coaxial. Neither is made from transmission line.

Colinears are very common on VHF Hi Band and higher frequencies, a popular series is the Cushman Station Master, a fiberglas pole with radials, the antenna elements and delay lines are inside a hollow fiberglas tube. The coaxial is a 1/4 wave rod above a 1/4 wave sleeve mounted over the mast with the transmission line run inside the mast. They were very popular on Lo Band, you used to see them on fire houses before the band fell out of favor. I have seen Hi Band ones too and several variations, rather a free standing colinear dipole array, some with radials.

A while back someone got the idea he could make a dipole out of coaxial cable. He removed the jacket covering 1/4 wave from the end and worked the braid back over the jacket exposing 1/4 wave of dielectric and center conductor. The braid ended up covering another 1/4 wave of the transmission line itself effectively making a coaxial antenna on the end of a piece of coax, a sort of all in one antenna system. BTW, this guy was a CBer, the one I made for 11M sucked, no wonder, he was a CBer. (;->)

Then along comes another bright idea, he found he could cut up the cable in similar fashion and fold the braid in two directions making yet another sort of antenna out of coax cable, the bazooka. Both the coaxial dipole and bazooka are very old designs, going back to the 20s I believe. Coaxial cable came much later and the guy with the knife born later still.

There is more to consider than simply using the 468/F formula to find the length for a half wave dipole, if it has insulation on it there is distributed C to consider, the physical length needs to be shortened. Then explaining how to make a bazooka is beyond the scope of this article and besides, it's getting long. The higher the frequency the more to consider so for now forget about making an 800mHz antenna out of coax. Do your homework, study study study, and when you learn a bit about antenna theory you can learn how to use one of the NEC antenna software packages and design your own.

One last thought, if you're going to use scientific terms, learn the definitions. Co-axial and co-linear are not the same thing.

 

[Al42]

"...how to make coaxial collinear antennas."

OK, let's clear up another misconception, ain't no such thing as a "coaxial colinear" antenna, it's an oxymoron. There is the colinear which is multiple half waves in phase and there is the vertical dipole, otherwise known as the coaxial.

True.

Neither is made from transmission line.

Actually both can be made from transmission line. There are many web pages describing 440 MHz and 2.5 GHz colinears made from transmission line segments, and the first coaxial design using a piece of RG-58U is probably older than I am.

A while back someone got the idea he could make a dipole out of coaxial cable. He removed the jacket covering 1/4 wave from the end and worked the braid back over the jacket exposing 1/4 wave of dielectric and center conductor. The braid ended up covering another 1/4 wave of the transmission line itself effectively making a coaxial antenna on the end of a piece of coax, a sort of all in one antenna system. BTW, this guy was a CBer, the one I made for 11M sucked, no wonder, he was a CBer. (;->)

The first one? Since we were using them on 2 meters long before hams lost 11 meters, it couldn't have been.

 

[nd5y]

a popular series is the Cushman Station Master

Station Masters were made by Phelps Dodge.
Cushman made test equipment.

 

[N_Jay]

a popular series is the Cushman Station Master

Station Masters were made by Phelps Dodge.
Cushman made test equipment.

Station Masters have been under a lot of brand names with all the buy-outs and mergers among the antenna companies.

Cushman is NOT one of them!

 

[califzeph]

Hi guys,

Oh good grief, where has all the logic gone? You're looking at the wrong end of the portable, think earphone. The whole pocket portable can be hidden in an inside pocket, right? Now you want to shut the darn thing up so the cop you're standing next to won't arrest you for carrying an illegal receiver. (Laws go back to the 1930s and deal with HF) The simple solution is headphones, he'll think you're listening to an iPod.

Oh, I'm not trying to break any laws... I'm in California where scanning is totally legal. I'd just like to look less obvious at the grocery store!

I DO use my iPod headset for my scanner. It works great for the Pro 97, which has a stereo output on it's headphone jack.

I just hate having the antenna sticking out! :lol:

 

[oregontreehugger]

I'd try a band-specific 800 MHz antenna (or even a stubby version). It sounds like that's what you're trying to monitor. That should be easier than a snake of coax cable following you down the grocery store aisles.

Something else useful on scanners without a stereo output on the headphone jack is the mono->stereo adapter at your local Radio Shack.

Or just buy a Uniden BC246T which is tiny, has stereo earphone output, and throw a stubby antenna on it.

 

[some1dial911]

It might not be what your looking for but use 2 of these(http://www.radioshack.com/product.asp?catalog_name=CTLG&product_id=278-116)
The antenna will then swivel to any direction, great for when your wearing it on your belt.[/img]

 

[califzeph]

Now, that's a great idea! And I'm half way there, I already have one of those right angle BNC connectors that I use in the car so the antenna doesn't bang on the windscreen! I'll just get another one and try it out!

 

[br0adband]

The Comet CH-32 'Miracle Baby' gets good reviews that I've noticed. You're not going to get much smaller than that one... but then again...

Antenex makes this particular "super stubby" and it even claims to have gain at the center frequencies in the 800 MHz and even the 900 MHz range on the particular models listed. Quite impressive, but the cost reflects the performance.

If you're looking for a stealth stubby, one of those look to be your best bets.

Hope this helps,

Paul

 

[Dubbin]

It might not be what your looking for but use 2 of these(http://www.radioshack.com/product.asp?catalog_name=CTLG&product_id=278-116)
The antenna will then swivel to any direction, great for when your wearing it on your belt.[/img]

Thats what I've been doing for years and it works great. Although I dont use those crappy RS adapters.

 

[Dubbin]

The Comet CH-32 'Miracle Baby' gets good reviews that I've noticed. You're not going to get much smaller than that one... but then again...

Been there, done that and its not that great unless you are are getting some really strong signals. It would be good for walking around the mall with though.