The title “General Antenna” refers to the fact that I am not right now concerned with 3-meter antennas, but with general antenna performance.
The title “General Antenna” refers to the fact that I am not right now concerned with 3-meter antennas, but with general antenna performance.
Over on the Big Talker thread(part 15 on shortwave) I said that today I changed the dipole antenna and had radically different results. Let me explain a few ongoing confusions.
WHEREAS most antenna books tell us to cut the length of an antenna to specific measurements, i.e., half wavelength, I found something different in a book by Edward M. Noll, W3FQJ, “73 DIPOLE AND LONG WIRE ANTENNAS,” where he writes, “Physical length is shorter than calculated wavelength by about 5%.”
Wow. 5-percent is a lot. Shorter.
AND another discrepancy has been noticed where some writers say the transmission-line should be a multiple of the wavelength, but another writer says the transmission-line should be an odd multiple of the wavelength.
Please close the gap on these seemingly different sets of instruction for calculating antennas. What say you?
RFB says
Theory And The Real World
“Please close the gap on these seemingly different sets of instruction for calculating antennas. What say you?”
Well this is an example of theory vs real world, and that the two rarely ever agree when it comes to application.
Though the theory gives us a starting point, a place to begin to apply that theory into real world situations.
Many things come into play regarding antenna lengths and feed line lengths. For example, the feed line fiasco. Much depends on the type of feed line being used..ie its physical diameter of the conductors, the velocity factors, the insulating materials and their effects, the frequency of operation, the power level, the losses per foot/meter factor, and of course the loading factors.
All of that will of course play around with the antenna and its length too. Other factors are environment conditions to take into consideration such as nearby objects, height above ground, ground conductivity, velocity factor of the wire, its size and diameter, mounting isolators and their insulating properties, frequency and power.
In the end…it is all one huge experiment once the starting point is established from the theory. I go by the rule of “Plus One”..that is to allow for more room, or more length, or more of “N”..then work backwards towards the starting point in the theory.
Example..If the theory calls for a 40 foot long wire for an antenna, I apply the Plus One so the length is 41 feet, then work it back.
Doing that has saved myself a lot of frustration and confusion, as well as always keeping in mind that NEC programs on a computer cannot account for real world conditions no matter what. The real world is always in a state of flux, change, along with other variables that continuously change in and around things.
RFB
Carl Blare says
For Example
Since today’s new dipole caused indoor reception from the indoor setup to become “over-charged,” I backed the power down at 13.560mHz until the radios stopped overloading at the front end, re-adjusted modulation, and noticed that residual hum, which typically exists in the house but not out in the field, became lower if I stood between my FM tower and the SW dipole.
The FM tower is a bamboo tripod supporting a cage-monopole at 101.9mHz which is the STL over to the SW transmitter. Simply killing the audio to the SW transmitter does not kill the hum, so there is some other interaction going on. I moved the bamboo tower 2-feet farther from the dipole and the hum diminished.
You live and try to learn.
radio8z says
5% Shorter
The reason a wire antenna is cut 5% shorter than the calculated length is because the velocity of the signal going down the wire is 95% the speed of light. This makes the physical wavelength on the wire shorter by 5% compared to the velocity in free space. The 95% figure is known as the velocity factor.
A transmission line which is an integer multiple of a half wavelength presents the same impedance to the source as is present at the load end of the line (antenna). This is not needed if the antenna Z is matched to the transmission line Z in which case the line length doesn’t matter except for loss.
Neil
Ken Norris says
Different ground connections
How are you grounding your devices?
Here’s a possible scenario: If you have one device’s power supply connected to an outlet (or strip plugged into an outlet) on one side of the room (or another room), and another device’s PS plugged into another outlet, where audio cables are connected between the two devices, there is a strong possibility there is a difference in ground imperfections (loss through leakage, creeping voltage across metal chassis’, etc., causing a potential between grounds such that you could be hearing a ground loop.
Solutions are to be sure all grounds are connected to the same ground grid, and at similar distances.
Transformers in cheap power supplies can also cause hum … this was the major problem with the TH tx I’m using now. The original PS was causing the problem, and the hum was not a 60 Hz hum and switching grounds had no effect. I replaced it with an older very heavy brick for a Hewlett-Packard printer, which I found in the used PS drawer at the local thrift store. The hum disappeared.
Truly great power supplies are rigorously tested to isolate internally generated noise. They shouldn’t be that hard to find. Ones like the one I’m using should never fail from loads like we’re talking about, so I expect mine to last at least as long as the transmitter. NOTE: It also doesn’t generate anywhere near as much heat as the original PS.
RFB says
Too Close For Hum-Free Comfort
“I moved the bamboo tower 2-feet farther from the dipole and the hum diminished.”
Ahh…the old and very common issue of too much signal ramming itself into nearby wiring. This is so true especially for setups indoors. 1/2 watt will wreak havoc on audio lines, other TX coaxes, other antennas, and even on metal cases of equipment and power cords.
And it may not even be the fundamental causing the ramming, it could be one of the harmonics. Is that 13 Mhz TX filtered on the output?
Even if it was, there can still be issues with too close proximity of things.
The opposite side of the room outlets is also sometimes problematic. Often one set of outlets on one side of the room are on one circuit and the others are on another, meaning one set of outlets is using one leg of the hots, and the other set is using the other leg, and most certainly will cause ground loop issues.
RFB