Dipole questions

[gdavid]

I just made a virticle dipole antenna using #14 copper wire and pvc for aircraft monitoring and it seemed to work well on the ground when I held it up by hand. The length is three half waves so the antenna is 11.5 feet. I mounted it on the side of a metal building with the bottom of the dipole 8 feet above the roof and 16 feet above the ground. Also I used rg8-U coax. This is strictly for listening so I wanted to get best reception I possibly could.
I am a little puzzled as to why the antenna is not working any better way up in the air as it worked when I held it up by hand.
A couple questions come to mind;
Is a three half wave dipole better at recieving than a single half wave?
Is there a problem with the relation to the metal building?
I can recieve the airport sometimes on 119 mhz but the auto broadcast on 128 mhz is always better.
I might cut it to a half wave size using the formula for 123 mhz or should I just get a discone and forget it?

Oh, one other concern, when I use my Radioshack handheld scanner, the reception is worse than it was on the ground but when I use an aircraft handheld transciever, its much better than the scanner but not what I had hoped... I guess..

Any suggestions from someone in the know would be appreciated!

Thanks!

 

[Al42]

Is a three half wave dipole better at recieving than a single half wave?

Each half is 3 half wavelengths? Or each half is 3 quarter wavelengths?

Is there a problem with the relation to the metal building?

Yes - the bottom of the antenna should be at least a full wavelength above the top of the building.

I can recieve the airport sometimes on 119 mhz but the auto broadcast on 128 mhz is always better.

The 128 MHz transmitter may be higher power, the antenna may be higher, or it may be located on a better path to you than the 119 MHz antenna.

I might cut it to a half wave size using the formula for 123 mhz or should I just get a discone and forget it?

For aircraft frequencies? Most discones are cut too high for that. I'd start with a half wave (total length) dipole.

Oh, one other concern, when I use my Radioshack handheld scanner, the reception is worse than it was on the ground but when I use an aircraft handheld transciever, its much better than the scanner but not what I had hoped... I guess..

GRE receivers (Radio Shack) are very prone to overload. Most of them work worse with an external antenna than with an internal antenna if there are any strong signals in the area.

 

[N_Jay]

I just made a virticle dipole antenna using #14 copper wire and pvc for aircraft monitoring and it seemed to work well on the ground when I held it up by hand. The length is three half waves so the antenna is 11.5 feet. I mounted it on the side of a metal building with the bottom of the dipole 8 feet above the roof and 16 feet above the ground. Also I used rg8-U coax. This is strictly for listening so I wanted to get best reception I possibly could.
I am a little puzzled as to why the antenna is not working any better way up in the air as it worked when I held it up by hand.
A couple questions come to mind;
Is a three half wave dipole better at recieving than a single half wave?
Is there a problem with the relation to the metal building?
I can recieve the airport sometimes on 119 mhz but the auto broadcast on 128 mhz is always better.
I might cut it to a half wave size using the formula for 123 mhz or should I just get a discone and forget it?

Oh, one other concern, when I use my Radioshack handheld scanner, the reception is worse than it was on the ground but when I use an aircraft handheld transciever, its much better than the scanner but not what I had hoped... I guess..

Any suggestions from someone in the know would be appreciated!

Thanks!

Make each side of the dipole 1/4 wavelength (like it is suposed to be)

 

[PluckyPleco]

Bigger isn't always better...

Make each side of the dipole 1/4 wavelength (like it is suposed to be)

While any antenna with two equal sides _could_ be called a dipole, the term is usually reserved for the half-wave dipole resonant at a given frequency. (1/4 wavelength on either side).

The two-equal sided antenna other than 1/4 wavelength sides is sometimes called a "doublette" (sp?).

"Bigger" isn't necessarily better when it comes to antennas...

Some people think that if a 1/4 wave on both sides is good, then a 1/2 on both sides is better... WRONG!!!

A half-wave dipole at resonance works so well is that it maximizes the potential of the incoming signal with no self-cancelation. (And no, I'm not gonna go into the Maxwell's equations or electrodynamics... ya want that go get a physics degree)

If you've got a 1/2 wavelength on either side, part of your received signal is cancelling itself out...

If you really want a 1/2 wavelength tall antenna, go 1/4 wavelength up, make a stub 1/4 wavelength long (out roughly 1/8 wavelength, up a tiny amount and then back in a 1/8 wavelength), then attach another 1/4 wavelength.

In simple terms... A signal hits the top 1/4 wavelength, then takes 1/4th wavelength time to get down to the bottom, where it is constructively added to the NEXT wave as it comes in, roughly doubling your received signal. (At the cost of a different received antenna pattern and a lower bandwidth antenna.)

Plec

 

[gdavid]

Thre half wave size

I made the antenna a total of three half waves ( 1.5 up and 1.5 down) I saw somewhere that an odd number would be good and the gain is better than than a half wave dipole. If an odd number of half waves (total from end to end) isn't always better, I will be glad to reduce the size because I can readjust the length for a little lower frequency and at the same time clear my metal roof a little better.

Thanks for the help!

Gary

 

[Al42]

If you've got a 1/2 wavelength on either side, part of your received signal is cancelling itself out...

Not to mention that the impedance is thousands of ohms, and that doesn't match any available coax too well.

 

[PluckyPleco]

While the following is somewhat wrong, it ain't off by much either.

For HF? Get "as long as you can get it" of a doublette "as high as you can get it," then feed it with ladder line to a balanced tuner.

Alternatively get a loop skywire as long as you can get it as high as you can get it then feed the same way.

My two cents worth.

Plec

 

[kf4lne]

Just cut each side of the dipole to 22 inches and be done with it. Run your coax away from the antenna at a 90 degree angle so the coax doesent become part of the antenna, ot better yet, get a peice of copper pipe, lowes has them in 24 inch lengths and push the coax up into it, connect the shield of the coax to the pipe and run another 24 inch length of wire up from the center conductor. Stuff all that inside some PVC and call it an air band antenna Once you have this antenna built (or whatever antenna you use) you will need to get it about 8 feet above the building. basically you will have a 4ft antenna 8 ft above the building and you should be able to hear better. I use an antenna similar to that for a air band receive antenna, but I made mine by folding the shield of the coax back over the coax so it is like being inside the pipe. The reason I say use copper pipe for the lower half is because after you spend all day trying to fold teh shield back over the outside of the RG8 you will realize it would have been cheaper and far less frustrating to just use the pipe

 

[srt1026]

question---your 'air band antenna', can this be modified by simply adding another length of wire to the center conducter to make it a multi-band antenna?? so you would have the pipe at the full length of the lowest freq, and then 2 wires on the top, one for the lower freq and another for the higher??

 

[USAPatriot]

On the other side of the question: There's a biological factor at work. I'm sure many people have noticed that when using portable radios and getting marginal signals, especially when there's static, that touching the antenna or sometimes just being close to it, will quite obviously improve reception. Moving closer to, or further away from such a radio, is a good experiment.

At one time I was a cardiology technician, so know a thing or two about electrical generation by the body...though not everything, not by a long shot.

With respect to the question above, I'd suspect that your body at the very least provided a better ground while you were holding it than what is being provided by it being mounted. You can test that by getting out the ladder and climbing up near to the antenna and see what it does. And by touching it. I'd recommend you do that in a cloudless sky though. Since the body operates on it's own bioelectric system it wouldn't surprise me if it can act as something of an amplifier, though in the case of the heart the power applied is measured in milliamps. Still, that 'may' add a decent boost to a passively received radio wave. Others can explore that question....I'm not qualified -Rod-

 

[N_Jay]

On the other side of the question: There's a biological factor at work. I'm sure many people have noticed that when using portable radios and getting marginal signals, especially when there's static, that touching the antenna or sometimes just being close to it, will quite obviously improve reception. Moving closer to, or further away from such a radio, is a good experiment.

At one time I was a cardiology technician, so know a thing or two about electrical generation by the body...though not everything, not by a long shot.

With respect to the question above, I'd suspect that your body at the very least provided a better ground while you were holding it than what is being provided by it being mounted. You can test that by getting out the ladder and climbing up near to the antenna and see what it does. And by touching it. I'd recommend you do that in a cloudless sky though. Since the body operates on it's own bioelectric system it wouldn't surprise me if it can act as something of an amplifier, though in the case of the heart the power applied is measured in milliamps. Still, that 'may' add a decent boost to a passively received radio wave. Others can explore that question....I'm not qualified -Rod-

Your body is conductive, and that is the effect you are having when near or touching an antenna. You are ecentaiily adding a "random" element to the antenna.

It has NOTHING to do with any biological factors or bioelectirc system.
You are in no way a "amplifier", just a conductive blob.

 

[Pro-95]

Conductive Blob..... LOL!

I have been learning more and more about antennas. I have officially reached the dangerous level.

1/2 wave dipole is as N-Jay described.

A doublette is usually the same thing but generally feed with ladder line rather than coax. But the terminology actually seems to be more the difference between RADICAL and FAROUT so more of era thing than a bonifide difference. Although depending on whom you talk to which is more the case. Check out LB Cebik's very informative antenna website.

I agree on the loop, if I put up another receiving only antenna it would be either the horizontal loop or a T2FD , tilted terminated folded dipole.

There are also a ton of people using really ridiculously long wire for receiving particularly in the shortwave arena.

 

[Al42]

On the other side of the question: There's a biological factor at work. I'm sure many people have noticed that when using portable radios and getting marginal signals, especially when there's static, that touching the antenna or sometimes just being close to it, will quite obviously improve reception. Moving closer to, or further away from such a radio, is a good experiment.

We use that trick to tune mobile antennas - if the reflected power goes down as you get near the antenna, the antenna is tuned to too high a frequency. (You tune it lower by coming closer to it.) Conversely if the reflected power goes up as you get close to it, it's tuned too low.

It's not a matter of the body's biological properties - a grounded piece of metal on a 20 foot long insulating rod would show the same effect. The body of the person tuning the antenna just happens to be a lot more readily available, and it's much easier to move around.

 

[Al42]

1/2 wave dipole is as N-Jay described.

A doublette is usually the same thing but generally feed with ladder line rather than coax.

A doublet is typically a folded dipole. It can be fed by balanced line (ladder line or twin-lead) or by coax with a balun.

(Doublette is a musical term, or it can refer to an artist's copy of his own work.)