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High Impedance devices and long wires

linearone

linearone

King of NY
Apr 2, 2005
929
303
73
Rotten Apple Trees
www.acboxes.com
Hams typically use long wire type antennas that folks tell me have an impedance of sometimes 1000 ohms etc.
If you have a High Impedance device , like a vacuum tube thats output is at lets say 1700 ohms, why cant you make a transmitter or amplifier with no lc circuit on the output , feed the wire through a shunt like a big cap to block the high voltage dc and have a 1700 ohm antenna 900 ft long (for arguments sake) and just fire it up?

this all assumes single band operation and a coincidence of the length of wire being close to resonant on the desired frequency.
 

linearone said:
Hams typically use long wire type antennas that folks tell me have an impedance of sometimes 1000 ohms etc.
If you have a High Impedance device , like a vacuum tube thats output is at lets say 1700 ohms, why cant you make a transmitter or amplifier with no lc circuit on the output , feed the wire through a shunt like a big cap to block the high voltage dc and have a 1700 ohm antenna 900 ft long (for arguments sake) and just fire it up?

this all assumes single band operation and a coincidence of the length of wire being close to resonant on the desired frequency.
Click to expand...

Harmonic suppression comes to mind.
 
Actually most hams use resonant antennas or close to resonant antennas that don't require much or just some easy matching, like with any typical antenna tuner. Very few hams use Longwire type antennas because they require such excessive fiddling with, require lots of real estate to even put up, are are highly directional, only one direction.

Wire antennas are used for convenience and are cheap and easy to make/buy and put up. Except for the lower bands like 80 and 160 which have very long wavelengths, wire antennas are a compromise. They are a comprise verses resonant beam antennas on a rotor which represent good gain and fingertip directionality.

I reckon your example will work Linearone but most people live in subdivisions and don't have 900' of land wherewith to stretch out such a longwire....plus there's the "only good in one direction factor."

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Actually, nothing says you can't do that. Lots of "if/and/but"s with it, but certainly possible. Doesn't really have to be an exceptionally long antenna, a 1/2 wave that's end fed has a pretty good sized input impedance, right? But then, that antenna's length is going to be one of the determining factors for what frequency you'll be able to operate on without changing it's length. So, you're sort of stuck with harmonically related frequencies... Hmm, sort of makes the frequency relationship with the ham bands make a little more sense, don't it (except for the WARC bands of course)??
- 'Doc

I'll be 'dipped', amazing what you can figure out when you look at it in the right way? Makes them 'magic' antenna lengths, like 1/4 and 1/2 waves sort of make more sense too.
 
You still need an LC circuit to tune the amp to the freq of operation. You can look at a typical Pi network as two separate LC sections. The first LC section tunes the amp to resonance and the second section matches that output impedance to the antenna.In theory it would be possible to have a simple L-network consisting of a single coil and a single cap to tune the amp to resonance and then direct couple an antenna of the proper impedance to the amp output. Possible yes,practical, no. Actually the very first spark transmitters were basically a high frequency AC generator coupled directly to an antenna. The freq of operation was pretty much determined by how well the antenna would radiate a certain frequency.
 
ajm1571 said:
I have a 10-80 inverted V that runs East/West and I seem to do "good" in all directions. NOT saying "good" is best by any means, but does work and I have made many contacts with it. just my .02
Click to expand...

An inverted Vee is not a longwire antenna. An inverted VEE tends to be pretty much omnidirectional unless installed at VERY high heights pretty much as does a regular dipole. A true longwire (notice it's written as one word) is not simply a long wire (notice it's written as two words). A longwire antenna consists of a very long wire that is at least one wavelength long at the lowest freq of operation.This means a true longwire must be at least 525 feet long on 160m. The use of an antenna that long on the higher bands will result in very pronounced directivity off the far end of the wire. For years I used a 600 foot wire antenna running to the north-west. It had good gain on 15 and 20m and worked quite well into Japan from my eastern Canada location. It sucked getting into Europe even though Europe is closer and a much easier target for me from here.
 
...It's all relative to what you might be used to. With my relatives, I'll stick to a typical dipole or loop, thank you.
- 'Doc

...odd, they feel the same way. My relatives I mean.
 

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