I vote for End-Fed Dipole
Hi MRAM! That's great you
could make a one stage 13.560 MHz
AM transmitter. I did that but had
trouble. It was an oscillator stage with
discrete parts. I modulated it just like
you are doing. I put a modulation
transformer in series with the power
lead. But it FMed like crazy! (And it
was crystal controlled.)
Eventually, I built the whole transmitter
from Carl's site, and it works fine, (no FMing) but
it is still just a bunch of parts hanging
together. It is supposed to run about 5 mW
to comply with Part 15. But I put it into my
QRP watt meter followed by a 50 ohm load
As I ramped up the voltage to the output stage,
I saw 200 or
300 mW on the watt meter. Modulation seemed to
be OK. Of course there is
no way to know how accurate this is. I don't
know the true output impedance.
Also, the transmsitter has no case or anything like that.
I want to get it into an enclosure, but
unfortunately, that is way down on the
priority list.
Very cool you did that with just one stage.
I would have been satisfied with just that.
Where did you find the time?
Bruce
P.S. I'm using a mag program
again. Hope this text looks OK.
Oh yeah, the 50 ohm load I
had on my 13 MHz transmitter
was a dummy load.
I'm sort of an O.C.D. guy, and
I would be too worried if I put
it into an antenna running that
kind of power.
Bruce
MRAM, we could discover more about your antenna if you can tell us the length of the horizontal antenna element and the length of the coax feed line. Is the center conductor of the coax connected to one end of the horizontal antenna, or somewhere else along the length of the horizontal antenna? Is the shield connected to anything at the antenna end, if so, what?
Yes Carl, that is the article regarding the end-fed dipole.
The antenna is cut for the bottom of the 20 meter ham band. That puts it less than 1/2 mHz from 13.560 mHz. But then it was made for the Ham band where I could test it.
I bought 100 feet of RG-174, a very thin coax. At about 16.7 feet from the end of the coax there are 4 ferrite rings through which the coax is wrapped about 6 or 8 times to form the choke. At the end of that 16.7 feet from the ferrite rings, attached to the center conductor of the coax, is another 16.7 feet of single conductor which forms the hot side of the antenna.
The antenna is sort of diagonal, hung from about 40 feet. In this picture you can see on the right the ferrite stack. Follow the cable to a strain relief which is were the shield ends and the single conductor is attached to the coax center conductor. The end of the single conductor is attached to an insulator and hanging from a spring attached to the guy wire. A string pulls the spring up the guy wire and is tied off to the tower.
With this antenna using less than 2 watts I made a CW contact over 300 miles away.
The bottom of the antenna is about 8 feet off the ground.
A dipole antenna typically has an impedance of around 70 ohms. It exhibits a low impedance because the overall length is 1/2 wave length and at the center of 1/2 wave length is where a low impedance exists.
My end-fed dipole shows an swr of less than 1.5 : 1 using RG-174/U coax which is 75 ohm coax. That indicates the feedline matches the antenna fairly well.
If this antenna were an end-fed 1/2 wave antenna the impedance would be very high. The result would be a great mismatch between the antenna and coax and a very high swr would result.
This seems to support the idea that the antenna operates as a 1/2 wave length dipole which has the appearance of being end fed even though by skin effect it's really center fed. In that regard I could agree that it's electrically center fed even though mechanically, it's not.
This antenna configuration has been known/used for many years, and often is referred to as a "sleeve" antenna.
In its simplest form it can be made by stripping the outer insulation from the coax to about a 1/4-wavelength distance at the operating frequency, and then pulling the coax shield/braid back down over the remaining outer insulation of the coax cable to form another ~ 1/4-wave conductor.
The open end of the braid is not connected to anything. The insulation on the 1/4-wave exposed length of the inner conductor doesn't need to be removed.
Folding the unterminated shield/braid down for about 1/4-wavelength over the unmodified coax produces a relatively high impedance to the r-f currents that might flow along the outer surface of the outer conductor of the incoming coax run, which provides about the same decoupling function as looping the coax around ferrite rings as shown in the PDF linked above.
Variations on this simple design include increasing the "gap" between the folded-down braid and the insulation on the outer conductor, to reduce losses in that path.
The radiation pattern shape and feedpoint impedance are close to that of a conventional, linear, 1/2-wave, center-fed dipole (other things equal). No physical "r-f ground" connection is needed or used, and in fact using one may change the electrical properties of such radiators.
But as MRAM notes, this is a center-fed configuration -- not an end-fed one. The antenna is not fed at either end, it is fed at its physical center.
Yes, a "sleeve" antenna sometimes referred to as a "coaxial" antenna popularized as a CB antenna with no radials.
The major difference being the "end-fed" antenna I'm using doesn't require folding the shield down over the coax to provide a sleeve. Rather, the ferrite choke provides the high impedance disconnect 1/4 wave from the center of the "dipole" and skin effect provides RF current on the outside of the shield as though it were a separate sleeve. That current is stopped at the choke point, 1/4 wave from the center "feed point."
The end result being the antenna has two equal length radiators electrically fed at the center even though the "connection" is at the end (or beginning actually) of the antenna at the choke point. That would be visually easy to accept if the coax were connectorized on the transmitter side of the choke, at the choke.
See what I started? Discourse ofcourse. 🙂
When one considers only the radiating conductors of this configuration to be "the antenna" (which is conventional), then the feedpoint is physically located where the coax inner conductor first extends beyond the coax outer conductor.
If the outer surface of the outer conductor of the first 1/4 wavelength of unmodified coax leading to the transmitter is de-coupled by some means (e.g. using a 1/4-wave sleeve or a ferrite choke arrangement) -- this makes the antenna a center-fed design.
The inner conductor in the 1/4-wave coaxial section starting at the feedpoint does not radiate, because the current along the outer surface of the inner conductor of that section is equal in amplitude and opposite in phase to the current flowing on the inner surface of the outer conductor, over that length.
A rare opportunity for me as we are in total agreement. 🙂
From my POV, you and I agree much more often than we don't.
Yes, end-fed seems to refer to the physical arrangement. We all seem to be in agreement.
Now let's tackle the folded dipole. Is it really a squashed loop antenna?