Maximum power transfer

Status
Not open for further replies.
120113-1633 EST

K8MHZ:

One needs a basic reference. Ultimately an isotropic radiator is a basic reference. Such a practical device does not exist for most radio measurements. A dipole can be defined relative to an isotropic radiator, and thus many measurements are referenced to a dipole.

See http://en.wikipedia.org/wiki/Antenna_gain

As you narrow the width of a radiated beam of energy for a given input power you will increase the power density, but unless you can perfectly collimate a beam, the energy density of the narrowed beam will diminish as the square of the radial distance.

When I was referring to the atmosphere having no effect I specified very low frequency radiation.

It is interesting that the results of the values given in the question produced a power density of 10 microwatts per sq-meter.

.
We (ham radio ops) use a reference to a half wave dipole in free space and label it as 'dBd', as opposed to 'dBi', gain over an isotropic model.

Yes, I see you addressed ELF, but the question about HAARP involved HF. Either way, until you get to the ionosphere, HF pretty much travels through the atmosphere with impunity, as does MW and ELF.
 

gar

Senior Member
120113-1702 EST

topgone:

Thanks for spotting my error. That is why I want my calculations checked. I misread the decimal point from the calculator when I multiplied 12.57 * 25 . Thus, 3.1416 * 10[SUP]-10[/SUP] is correct, and this makes the power density about 100 microwatts instead of 10 per sq-meter.

.
 

Joethemechanic

Senior Member
Location
Philly Pa burbs
I gotta go for a bit, but think about ERP backwards for a second.

I used a 175 watt transmitter on SSB, which is pretty much a 40-80 percent signal (compared to an unmodulated AM carrier) due to the way it modulates the signal, connected to an antenna made entirely out of old coax I got off the top of our county building and made a contact to Japan with it. I had no amp, just the power from the back of the radio into the 'double bazooka' antenna strung between some trees in my back yard. The station was about 9,600 miles away. Think of how miniscule my signal must have been. To me, that is just as awesome as the big iron.

If you want to do the math, figure 155 watts into a 2dBi (2dB better than an isotropic radiator) gain antenna with a path of 9,600 miles. I would be interested in what that would work out to.
That was on the 20 meter band?
 

Joethemechanic

Senior Member
Location
Philly Pa burbs
Yes, and at the solar minimum. At the time, 20 was the only DX band useable on SSB. And when you consider that it was at least a double bounce, the path loss was much greater than a line of site contact at the same distance.
From what I am understanding you built a stationary antenna out of old coax with somewhat directed radiation? Was this old 75 ohm stuff?

What is CW on the longer wave bands like 80 meters like now? I can remember some low wattage stations with antennas built from 300 ohm flat antenna wire working a big part of the world.

The only problem I had with CW is I really was never any good at transcribing it. There for a while I was ok with the key, but I was never that good. Now I have forgotten almost all of it.
 
From what I am understanding you built a stationary antenna out of old coax with somewhat directed radiation? Was this old 75 ohm stuff?

What is CW on the longer wave bands like 80 meters like now? I can remember some low wattage stations with antennas built from 300 ohm flat antenna wire working a big part of the world.

The only problem I had with CW is I really was never any good at transcribing it. There for a while I was ok with the key, but I was never that good. Now I have forgotten almost all of it.
The antenna was a 'double bazooka' first used in the 50's for radar. Using coax for the elements makes it broad banded over a wire dipole. There is really no advantage over a regular dipole other than some additional bandwidth, and it isn't much. It's also a single band antenna no matter what. I built it because 1) I got the coax for free. 2) The antenna is notoriously difficult to construct and tune properly so I was in it for the challenge. I ended up wasting about 100 feet of coax before I got it right. 3) It was a fun spring project for my daughter and me. She was 11 or so and had just got her first ham license.

The coax was 50 ohm and was feeding vertical antennas on the roof of the county building. Our county isn't rich, so once they got the money for new materials the local ham operators got together and did the labor of upgrading the antenna systems. I got a half a van full old coax to play with for my efforts.

Now that the sun is back on, all the bands are hot at one time of the day or the other. CW on any band is an advantage. I know Morse Code, but seldom use it.

You can make a folded dipole out of twin lead that will handle hundreds of watts, so long as it is resonant, or close.

For 2 meters you can make a vertical called a 'j-pole' out of twin lead. The antenna is good for 50 watts or more, but black tape isn't.

I had a student that just got his license and wanted to play with his 2 meter radio but didn't have an antenna. I had an old j-pole that I brought over and we hooked it up in his garage. The radio had three settings, 5, 10 and 50 watts. We managed to get into all the local repeaters with 5 watts. 10 got us a couple more. Now I wanted to see if I could hit a repeater that was about 70 miles away so I turned it up to 50 watts and started calling a friend of mine that could usually be found on that repeater.

Jeff, my student (and now good friend) was watching and just as dead pan as could be asked, 'Is it supposed to be on fire like that?' I looked up and some black tape in the middle of the antenna was on fire, flames, smoke, the whole bit.

I put the fire out and sheepishly scraped the gook off the antenna and cleaned it up. Once the tape was gone, 50 watts was no problem.

OK....class back in session. The tape was covering a tuning notch because I was going to use the antenna on a 5 watt handheld outdoors and didn't want water to get into it. At 50 watts the voltage across the notch is such that the glue on tape will conduct a bit and cause an arc-over and ignite said tape. Applied Murphology dictates that if said fire is not sufficient to burn down a building, it will happen to a teacher in front of a student he is trying to impress.....

Morse Code is still very popular, but there are other digital modes that are great for weak signal and can be done with a computer and a free program.
 
Status
Not open for further replies.
Top