dipole debugging

[scowl]

I have a 60 foot dipole. 30 feet in one direction and 30 feet in the other direction. To test it I ran three feet of coax to my receiver while I was on the roof.

Connecting one pole to the center conductor gave me good reception. Connecting the other pole gave me identical reception. However connecting them both (center and shield) like a proper dipole showed no measurable improvement. I might as well have hung just 30 feet of wire and be done with it.

Is this because dipoles are most effective in the narrow frequency ranges that match the wavelength of the antenna?

 

[Boombox]

From my understanding of dipoles, they will receive well on a wider range of frequencies than they will transmit. They are tuned more for transmitting, than they are for receiving.

 

[LtDoc]

What frequency did you do that listening on? That antenna would be for the 7.5 - 8.5 Mhz region and that's where it would 'hear' best. The further you get from it's resonant frequency the less it will 'hear' until you get to a harmonic of it's design frequency where it will 'improve' again. That difference in 'hearing' wont be 'huge' but there will be a difference. What you experienced with that one side other other only being connected is normal with any antenna with two terminal. You're right, a resonant antenna will have a wider usable frequency when receiving and that's because resonance isn't as important as when transmitting. Receivers tend to be more 'forgiving' that transmitters.
- 'Doc

 

[SCPD]

I have a 60 foot dipole. 30 feet in one direction and 30 feet in the other direction. To test it I ran three feet of coax to my receiver while I was on the roof.

Connecting one pole to the center conductor gave me good reception. Connecting the other pole gave me identical reception. However connecting them both (center and shield) like a proper dipole showed no measurable improvement. I might as well have hung just 30 feet of wire and be done with it.

Is this because dipoles are most effective in the narrow frequency ranges that match the wavelength of the antenna?

Welcome to HF radio. Ah this brings back memories. It seems logical right? Legs of 30ft + 30ft = 60ft and that should double your performance!

First HF lesson: Just about ANY conductive object (including coax) can and will act as an antenna

Second HF lesson: Buy, borrow or checkout a copy of the ARRL Handbook. You can learn a lot by reading this book.

You can probably get one that's a couple of years old for $1 at Goodwill or a used book store. (The year really won't matter. These principles haven't really changed much in the last ten years.)

 

[Token]

I have a 60 foot dipole. 30 feet in one direction and 30 feet in the other direction. To test it I ran three feet of coax to my receiver while I was on the roof.

What kind of connector was on the coax and the radio? Possibly, what kind of radio was it?

Connecting one pole to the center conductor gave me good reception. Connecting the other pole gave me identical reception. However connecting them both (center and shield) like a proper dipole showed no measurable improvement. I might as well have hung just 30 feet of wire and be done with it.

Is this because dipoles are most effective in the narrow frequency ranges that match the wavelength of the antenna?

Theory aside, just talking about real world applications here:

Connecting them both showed no improvement that you could detect under your particular testing criteria. What frequencies did you check on? What signals were used in your test? How are those signal sources physically oriented with regards to your antenna? What was your criteria for “better”, was it just signal strength? Was it signal to noise?

That antenna should have been a half wave dipole at about 7800 kHz. This means this is about where its peak performance will be. As you move away from this frequency the antenna will start to act more and more like a random wire. Dipoles are most affective across a specific frequency range, and its multiples or harmonically related bands (this means half the frequency, 2x the frequency, 3x the frequency, etc), depending on their particular physical and electrical characteristics (might be a narrow range of frequencies for some designs / builds, not so much for others). They will work away from those frequencies, just not as efficiently. They are seldom “worse” than a random wire, but they may not be “better” except in certain bands.

T!

 

[k9rzz]

As a general rule of thumb, you've got to give an antenna more than 10 minutes to prove itself. You might just find that one configuration will have a lobe of max reception on Europe on 19 meters, and another be better into Africa or be best below 6 Mhz. Have faith in your work and keep playing around, that's where the FUN is!

 

[majoco]

I really don't understand why people erect dipoles for SWL unless they are particularly interested in a specific band. An OCFD covers a multitude of frequencies with the same polar diagram as a dipole.

 

[zz0468]

They will work away from those frequencies, just not as efficiently. They are seldom “worse” than a random wire, but they may not be “better” except in certain bands.

What he said...

In my personal experience, I've had single band coax fed dipoles that worked great on their fundamental and 3rd harmonic, and were absolutely miserable elsewhere. So, I never advocate using a dipole for SWL applications.

There is an exception, and that is a dipole fed with open wire line. Go to a tuner, and it'll be a nice multiband antenna. I've even used open wire dipoles to a toroid balun, to a receiver and seen good performance across a wide band.