problem with resonant frequency

[foTONICS]

Hey all,

I work at a school lending out materials and offering advice whenever I can. A student came up to me the other day and was asking about folded dipole antennas. He was constructing a antenna that he built from a schematic in a textbook (This was a few weeks ago so I don't have any dimensions or data). He said he followed the design perfectly and even had a machine etch it onto a PCB.

His problem was his resonate frequency. I believe he was shooting for 15 MHz and ended up with 13MHz as his resonant frequency. Now I know that small little things like transmission line length, self-induced capacitance, etc would cause a small shift in resonance but 13%?

We talked about ways to solve it (i.e. loading coils and adjustable capacitors) but everything just came back down why such a huge gap in resonance. Now I'm only a 3rd year student so my antenna knowledge is limited so I was wondering if any of you guys had any input. I love to trouble shoot and solve problems so this has been eating at me.

Here are a couple of ideas that I have had that may cause a problem:

1. Would a small crack or break in the short between the two ends of the folded di-pole affect the antenna and reduce it to a regular di-pole antenna

2. He was using a comm analyzer to measure the center frequency, could a setting or adjustment error make this 13% possible?

3. These analyzers, in addition to the antenna being tested, are in a small lab about, lets say 25' X 25' and packed to the brim with lockers and computers, not to mention the labs walls are concrete blocks. Could any kind of reflected waves induce a sort of out of phase component knocking the antenna of its center?

 

[foTONICS]

On a side note he did mention that he was having a problem with load balancing, but as I recall this should only have a negative inpact on the propagation of RF energy, not its center frequency

 

[duke37]

A 15MHz half wave folded dipole in free space would be about 10m long. Are you sure this was constructed on a PCB?

15% low seems a bit much but is feasible, there will be some dielectric between the parallel lines of the dipole which will drop the wave velocity.

Concrete blocks are unlikely to have much effect but metal objects within a few wavelengths will cause variations.

 

[foTONICS]

A 15MHz half wave folded dipole in free space would be about 10m long. Are you sure this was constructed on a PCB?.

I held the actual antenna, I could be wrong about the 15MHz, might be 1.5MHz. I'm sure it was in the megahertz band though

 

[john monks]

You must mean 1.5GHz the length is about one wavelength long. So the total length is going to be about C (the speed of light) divided by the frequency in hertz.
If the antenna Is about in the center of the room the walls, lights, and floor will have little affect. The fact that the antenna is etched onto a pc board will have significant affect, maybe 13%.
My advice is to fined the resonant frequency of is existing antenna, make a new antenna out of similar pc board, scaling it up or down, probably down, for the resonant frequency he wants.
1. No.
2. A loose connection or a mismatched coax can cause error.
3. Almost none.
There is no easy answer to the question about the 13% because the pc board, the dimensions of the conductors because of skin affect, the layout, and many other things affect resonance. At this point we don't even know if the antenna appears capacitive or inductive. It seems like every a mature radio operator and broadcaster has face this same problem and ended up modifying his original design. Sure this can be figured out with college physics. The physics of electro-magnetism are well known. But the math becomes so complicated that it may take a whole day to come close to the right design if he didn't make a mistake.

There are many good designs on the Internet. If he were starting from scratch I would suggest that he copy one and modify it with a loading coil or capacitor as needed.

 

[davenn]

I held the actual antenna, I could be wrong about the 15MHz, might be 1.5MHz. I'm sure it was in the megahertz band though

1.5 MHz is even longer .... 100m long for a half wave dipole

its must have been, as John suggested, 1.5 GHz, there a halfwave dipole is 10cm long

for wavelength 300 / freq (in MHz) eg 300 / 1500MHz = 0.2m (20cm)
20cm / 2 = 10cm = half wavelength


Dave

 

[john monks]

Davenn, you are quite correct.
Most dipole antennas and folded dipole antennas are 1/2 the wavelength, that is from end to end.
To be more precise the length of the dipoles will be 299800000*0.5*0.95/1500000000 or about 0.0949 meters.
The 0.95 comes from what is called "end effect"