Ground - plane not sure ?

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usnasa

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to you kb2vxa lol sorry so what would you suggest i do should i solder the wires on each corner dont cross them as you suggest ? straight out
 
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kb2vxa

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Please make up your mind whether you're going to PM me or put it on the forum but not both. I don't run fast enough to keep up with you as it is and now I feel like a ping pong ball in a washing machine. Remember that song by Tommy Roe? (;->)

Dizzy, I'm so dizzy my head is spinning
Like a whirlpool it never ends
And it's usnasa makin' it spin
You're making me dizzy...
 

rankin39

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The metal sheet under the antenna used for the ground plane doesn't have to attract magnetically. You can use ordinary aluminum foil. Works just fine indoors.

Bob, W0NXN
 

ARCA-Dave

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800 mhz ground plane

Hey all, Dave here. I am now feeding this sight ( http://sc.scanamerica.us/index.php) stuff around Summerville , Dorchester County and the SCHP around Charleston and the tri county area. Check it out, it is an awesome sight....I use a Radio Shack Pro 2096 and I had tried an outside groundplane, but have had overwhelming success with the back of the set telescoping stick collapsed all the way down. I get over 95% signal on all of the control channels in the area on this setup...not bad for a newbie...Take care and enjoy...Dave

PS...I do have the outside ground plane, does anyone know how I would tune it for the 856mhz area to better pull in those weak signals insted of using the back of the set antenna? I can do some cutting or I could go out and buy a $40 ground plane from ebay........any suggestions??

Here is a pic of what it looks like and where it is mounted.
 

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usnasa

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IM SORRY ABOUT THAT KB2VXA and sorry to bother you but im confused
here is what my set up looks like now, funny thing is my NYC DOITT 800 frequiences are coming in clear
http://www.radioreference.com/apps/db/?sid=129
but my 800 PORT AUTHORITY channels are very weak
http://www.radioreference.com/apps/db/?sid=408
here ARE picS am i better off bolting the wire radials to the ground plane or just leave them looped onto the antenna .


Please make up your mind whether you're going to PM me or put it on the forum but not both. I don't run fast enough to keep up with you as it is and now I feel like a ping pong ball in a washing machine. Remember that song by Tommy Roe? (;->)

Dizzy, I'm so dizzy my head is spinning
Like a whirlpool it never ends
And it's usnasa makin' it spin
You're making me dizzy...
 

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N_Jay

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The radials need to be attached the ground, not the main element.
 

usnasa

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so what your saying it should be bolted or solder on the metal baking sheet in four corners straigh out correct and thanks
 

kb2vxa

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I don't mean to belittle you or anything but your misunderstanding gave me quite a chuckle. You reminded me of when I started out like a copper spider spinning antenna webs and later on as a TV serviceman seeing tin foil hanging on rabbit ears set top antennas. You'll get there if I don't pull my hair out first so again not to belittle you but seriously the first thing you need to learn is how to read more carefully and follow instructions. Oh we all have this problem at least once in our lives, I'm just passing on the advice of one of my school teachers.

Still don't expect too much from so little, it's not the best of antennas no matter how you look at it but the best you can do under present circumstances. Something just rang a bell in the back of my head, sometimes moving the antenna a bit makes a difference. Without getting all technical there are radio dead spots (I've often parked my mobile in them) so you may just have to move the antenna around a bit. It's like a tennis racket, you have to find the sweet spot. Heh, I mentioned that because it's exactly what I did about an hour ago and the State Police sound a whole lot better. Here's another crappy photo but at least you can see the mounting plate and radials now.

For you Dave;
"PS...I do have the outside ground plane, does anyone know how I would tune it for the 856mhz area to better pull in those weak signals instead of using the back of the set antenna? I can do some cutting or I could go out and buy a $40 ground plane from ebay........any suggestions??"

Suggestion first, leave it alone. It's a 1/4 wave VHF Hi Band antenna which just happens to be 3/4 wave on 450MHz UHF so it works best on those bands. If you cut it to a 3" 1/4 wave or 9" 3/4 wave on 800MHz it'll work much better there but nowhere else. You say you use the "stock" set-back whip antenna fully collapsed for 800 and I'll bet a few donuts it's 9" long that way. That's pretty darn good without complicating things so as they say in the maintenance department, if it ain't broke don't fix it.
 
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Mike_G_D

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De plane...de...oh whatever, you know...

usnasa,

Here's some basics you should try and commit to memory:

Most smaller vertical antennas "attempt" to "be" a 1/2wave at their designed for center frequency. For the most part, the 1/2 wave design is the most common and overall useful "core" of most vertical and horizontal antennas. I am WAY over simplifying here but I am trying to "stick to bare bones basics" for usanasa's benefit. I know there are exceptions, etc., etc. Ok...

Now, very simply put, you get the wavelength of a particular frequency by dividing the speed of light by the frequency. A simplified form of the equation which works for frequencies in MHz is

300 divided by the frequency in MHz (300/Freq. in MHz) = wavelength in meters (in free space with no air, atmosphere, etc.).

So, for example, 300/150MHz equals 2 meters. A meter is approximately 39.37 in. So you get 39.37 x 2 = 78.74in. That's a FULL WAVELENGTH at 150MHz.

Now, because we are shooting for 1/2 wave to make our antenna we go for 1 meter or about 39.37in.

The 1/2 wave design is most commonly realized as a dipole which has two elements each a 1/4 wave long and fed by the lead in at the center. If you use coax you would connect the center wire to one half of the dipole and the shield to the other half (I am not going to go into baluns at this point - just trying to get him in the general ball park). You can mount this vertically or horizontally but for typical land mobile radio usage we go vertical. The center feed makes the antenna match reasonably well to common coaxial impedances (it comes to about 70 ohms and works ok for either 75 ohm or 50 ohm coaxial cable).

Common variations of this are the coaxial dipole, the end-fed 1/2wave, the so-called "ground plane", and the 1/4 wave whip with a ground plane.

The most common type you run into for mobile usage is the 1/4 wave whip. To work properly, the 1/4 wave whip needs to have its "second half" to make a full 1/2 wave. That "second half" is what is referred to as "the ground plane". This is provided by the vehicle's metal body (say the car roof top or the trunk lid, etc.). Basically, you want that ground plane to be big enough to work at the frequency of choice if at all possible. The lower the frequency the longer the wavelength, the longer the antenna and the bigger the ground plane needed for ideal performance.

For our example at 150MHz, we would need the 1/4 wave whip part to be about 19.5in (roughly half of 39.37in) and the ground plane to be at least that long measuring out from the center. If the ground plane is larger that's usually ok. But if it is below the desired length performance will worsen. A car roof top or trunk lid will usually provide a decent enough ground plane for most land mobile radio frequencies. So the common 1/4 wave whip is often used in such setups.

If you lack a good ground plane, say because you have a fiberglass top as in some motorhomes and on boats then you should provide one either by using radials that are of the correct length attached to the shield of the coax at the base of the 1/4 wave vertical element (shield is attached to the radials and center conductor is attached to the vertical element). The more radials the better but 3 or 4 should suffice. They can either be sloped down at an angle or spread out at a 90 degree angle from the base (technically, a 30 to 45 degree angle should provide a better match at 50 ohms but so many other factors come into play in practical real world setups that this is usually not a concern) or even just dropped down vertically (so you get your 1/2 wave vertical dipole).

Other options when you lack a ground plane to mount your antenna on are to use the coaxial dipole, which is basically just a normal dipole mounted vertically with the lower half being a "tube" wherein the coax is fed through it to connect to the middle, and the end fed 1/2wave wherein the lead is attached at the base of a 1/2wave continuous element and a matching network is used to match the impedance to that of the line. Both of these types of antennas do not need a ground plane as they are a full 1/2 wave long.

The other common antenna used in base station use is the so-called "ground plane" antenna. This is just a 1/4wave whip with radiating elements to make the ground plane just as described above.

What you are trying to do is use a mobile antenna inside the room and give it a decent ground plane at the frequency of interest. For 860MHz we get 300/860 = 0.349meters which is 0.349m x 39.37in/m = 13.73in (full wave length). 1/4 of this would be 13.73in/4 which gives you 3.43in. So your vertical element needs to be about 3in in length and your ground plane needs to be at least that long. So the table you are currently using looks pretty decent for 860MHz. At the lower frequencies, of course, it gets more limited. But any ground plane is better than none for a 1/4 wave whip. Now those numbers are ideal numbers in free space with no air/no atmosphere. Hear on Earth the speed of light slows down slightly so we get slightly different numbers. But, especially for receive only purposes, this gets you in the ball park.

Just try and remember:

1) Wavelength in meters = 300/Frequency in MHz.

2) 1 meter = approx. 39.37in.

3) 1/2 wave antenna is core design.

4) 1/4 wave antenna needs its "second half" or "ground plane" to work properly.

5) The "ground plane" can be radials made of wire, metal rods, etc. or a large base of metal that has at least enough surface area to accommodate the correct "length" of ground plane radiating out from the base of the antenna and is coupled to the coax shield.

The magnet mount antenna effectively "attaches the ground plane to the coax shield by means of "capacitive coupling". This is not a direct connection (in terms of DC) but it works at radio frequencies - just trust me here as to explain this further wouldn't be practical at this point and add way more to an already lengthy post on my part.

Multi-band antennas use various "tricks" to try and get overall acceptable performance across a large frequency range. Some designs work better than others but overall, most are a sort of compromise versus having a tuned antenna dedicated to the frequency of interest. Again, explaining this is beyond the scope of this post.

You should really try and memorize these basic points as they will help you get a handle on all this "antenna and ground plane stuff".

-Mike
 

ARCA-Dave

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WOW that was and awesome explaination Mike

Hey Mike, Dave here. That was awesome the way you explained that. This confirms my speculations about my original question too. I was wondering how long (if I should even try) to either cut the elements on this ground plane on my rooftop or fabricate a new antenna. (SEE my previous pictures). I recently fabricated this (see pics below) of my "800 mhz mobil groundplane". I slip the vertical element into the magnetic base and tighten the set screw and down the road I go. The vertical element is 3 & 3/8 tall and the "legs" or the 4 ground plane radials (and let me know if I misnamed them) are 3 & 1/4 each. It is not pretty but I would imagine it should do pretty well in the center of my roof. I haven't really had time to take it out and experiment but what do yall think other than not too pretty? It is just an so-239 socket. And as for my outside ground plane, maybe just cut the elements or leave it alone and use the back of set telescopeing antenna? Thanks for the help....Dave
 

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Mike_G_D

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Dave's ground plane

Dave,

Well the dimensions look good though I am not clear on how you "mount" the vertical element. I don't see it in the picture but I assume you add it during usage. Otherwise it should be fine. If you do mount it outside I'd add some caulking or some type of weather seal around the coax connection to keep moisture out. Also, SO-239 connectors (and the PL-259's they connect to) are rather poor choices at 800MHz. If you get a chance, try going to an N connector or maybe a SMA. Even a CATV F-connector might be a better choice. But it will probably work ok for now as is. Mainly just weather seal it somehow.

And I agree with others that you might just keep the RS ground plane as is and use it for general purpose stuff; especially given that making an 800MHz ground plane antenna is, as you have found out, pretty easy to do!

-Mike
 

ARCA-Dave

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Pic of the mount

I guess I should have ran out in the cold...at midnight 30 no less...(grin) and taken the picture before it got so cold. Here is the way I mount it to the magnetic mount. I just slip it into the top and tighten down the screw. I don't know if the vertical element should go up higher than the "legs" or it is ok like they are? (see previous posts of my outside groundplane). I was thinking they may block reception from the vertical element, but I really am not sure. If I did it that way, the "legs" would have to almost extend out horizontal to keep them from hitting the car roof. Anyway here it is, and normally it is in the CENTER of the roof, but I moved it closer for picture purposes....Thanks again for any help, Dave
 

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Mike_G_D

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Ummm....no...

Uh, no, don't do that!

Look at that VHF ground plane antenna you have. Think about that and how it is connected to the coax. That's how your home brew antenna should look - just smaller.

The radials are coupled to the coax shield and the vertical element is connected to the center conductor.

What you have there, as pictured, is, well, I'm not sure what to call it! Anyway, for your car, using the mag mount, forget about making a ground plane - the mag mount is using your car's roof as that. All you need is the vertical element. To use your picture - get rid of those "radials" (they're not functioning as such in that configuration anyway). Just put that 3in straight wire in and that's it.

For your house, then you would want a ground plane. Again study how that VHF ground plane antenna that you already have on the roof is made and just think make the dimensions smaller (3in radials and 3in vertical).

If you had your SO-239 connector - solder your radials to the outer part (this is what I think you did right) BUT put the center vertical element into the center pin on the opposite side from where the coax would connect. In other words - make it like the VHF antenna but smaller - again, think carefully.

Center conductor---> vertical element

Shield-----------------> radials

vertical element should be ABOVE the radials.

-Mike
 
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ARCA-Dave

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You guys are awesome

I just wanted to let you all know that your'e a lot smarter than the bunch of guys I talk to on the Ford-trucks.com websight about my F-350! Here is the ORIGINAL groundplane design I found a long time ago and "tried to copy" that I had to modify to clear the roof on my car. I guess I should just stick a single piece of copper or like I had heard, a stainless steal bycicle spoke down in the mag mount to make an antenna for my car. Anyway, here is the original design. I will go ahead and build one I think, and try for my house scanner. Another "no no" I got an idea for was connecting the little goundplane and big groundplane with a "T" connector and run a single length of RG8u coax into the house? Bad idea or not? Thanks again for all the advice....Dave
 

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N_Jay

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You don't need a ground plane on your car.

The roof IS the ground plane.

Just cut your radiator to length.
 

nexus

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I'm just speechless....... good god... (shakes head) I thought I saw everything, boy was I wrong.

I guess I should have ran out in the cold...at midnight 30 no less...(grin) and taken the picture before it got so cold. Here is the way I mount it to the magnetic mount. I just slip it into the top and tighten down the screw. I don't know if the vertical element should go up higher than the "legs" or it is ok like they are? (see previous posts of my outside groundplane). I was thinking they may block reception from the vertical element, but I really am not sure. If I did it that way, the "legs" would have to almost extend out horizontal to keep them from hitting the car roof. Anyway here it is, and normally it is in the CENTER of the roof, but I moved it closer for picture purposes....Thanks again for any help, Dave
 
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