If you knew someone with a large throw lathe and a screw turning attachment, you could cut a grove in PVC that would make it easy. Would space between the windings give a better capacitance? Which might once again improve the efficiency???
I really need to talk to my boss about buying one of these so I can play. It's been 40-50 degrees here all month, and I could have been up on the tower fiddling to get ready for the spring semester...
Which sounds like a punk rock band, but seriously folks, the one shortcoming of that is that it would still be hard to connect to the coil turns. Better, but not ideal.
This Texas bugcatcher coil runs around $250, reasonable considering the quality, but a bit steep for casual experimentation.
Experimental broadcasting for a better tomorrow!
There are Threaded PVC Nipples Which sounds like a punk rock band
Bwaahahaha! I just about spewed coffee on the mixing desk! 🙂 Funny!
This is the concept behind the "screwdriver" antenna. The coil is wound with uninsulated wire along a spiral groove to keep the wires separated. A spring-loaded contact ring contacts the wire. The outer sheath containing the contact ring is driven up and down the coil by a reversible motor (originally an electric screwdriver - thus the name).
Do a google search on "screwdriver antenna" to see what is available to hams. There is even at least one model (I think) made for 160 meters (about 1800kHz) that could possibly work at the high end of the AM band. These antennas are intended for mobile use, so are small and within part 15 limits.
A major advantage for us would be the remote tuning capability. After mounting it high up, you can fine-tune it from the comfort of your easy chair.
The disadvantage of the standard offerings is that the coil tends to be very long with respect to the antenna length. This allows a wide tuning range at the expense of antenna length.
This would be a great opportunity for someone to design a version targeted to part 15 AM. I envision a shorter, possibly larger diameter coil with a longer antenna (keeping within the 3 meter limit). It should tune 1500-1700kHz.
The original screwdriver antenna plans for home construction were complicated (the plans are on the web somewhere). A strip of bronze finger contact stock was formed into a ring for the coil contact. You see this stuff along the edges of electronic cabinets for EMI shielding. One very important hurdle is to make the whole coil assembly absolutely waterproof. This requires a rubber boot or some such over the coil and contact sheath assembly.
This is a fun technical idea, but I think it would be overkill for part 15ers. We don't change frequencies all the time as the hams do. Nevertheless, the remote tuning capability is really attractive when the antenna is mounted waaay up on a tower!
But if antenna system Q is too high, it will reduce the amplitude of the AM sidebands that are radiated, and the audio at the receiver can sound quite muffled -- because the higher audio frequencies have been filtered out. High Q also makes the antenna system more difficult to tune to, and maintain system resonance, and makes the antenna match more sensitive to its mounting environment. So as in many things, a compromise may be useful.
Rich, I agree with your analysis, except for the part about adding a resistor 🙂
It seems the price we pay for achieving the lowest possible ground loss resistance is reduced bandwidth due to higher Q. In reality, I don't think this is a real problem. High frequency pre-emphasis can overcome reduced bandwidth to some degree. And, as you said, most of us don't have such a perfect ground.
This brings up a question to all in the forum. Has anyone actually experienced audio muffling due to their antenna installation? This is dangerous territory. It is subjective, and should be backed up with a comparison of the transmitter audio in a lower Q setup. It would probably be better for this to be discussed in a separate thread.
So if we have more taps, like one every turn, then can we get to where the antenna length adjustment is less important? That way once the antenna length is close, it colud be soldered to prevent corrosion and all final tuning could be at the coil. You could even pull the coil apart and use the extra wire to forms taps around the required point every quarter turn. Maybe a quarter turn is far too close, but every turn or every half turn is certainly withing the realm of workable.
What about the extra wire at the end of the coil, doesn't it effect things as much as another antenna would? It is an unterminated conductor, so it should radiate.
Hi Greg,
Extra taps around the circumference of the coil would indeed reduce the need for adjusting the antenna length. I once experimented with a coil that had taps every two turns like the original, but at the top I tapped the top two turns every 45 degrees for a total of 16 taps. With this arrangement, you connect the antenna to the midpoint of the top two-turn taps and then select the best bottom tap. Then move the antenna wire to various taps at the top. This arrangement allows coarse tuning with the bottom taps and fine tuning with the top taps.
I'm somewhat of a perfectionist, so I judged that the results with the top taps weren't quite as good as fine-tuning by adjusting the antenna length. In reality, the difference is probably not going to be noticeable! The real problem is that the SSTRAN antenna already has a lot of taps and asking people to add 16 more taps at the top is troublesome. I think it is easier for the average builder to implement the copper pipe length adjustment than to add 16 more taps. I may be wrong.
One more question that I was thinking about, this one aimed at the antenna.
Realizing that part 15 AM is far from an ideal antenna length, I was wondering if making the antenna an even multiple of the wavelength would be of much help. It looks like we are skirting the 1/64 wave size, with 1500Khz being over the length (about 3.1 meters), and 1700 being shorter (about 2.75 meters)[unless my math is wrong]. So if we picked a frequency of say 1640, my math shows that as being about 2.85 meters long for a 1/64 wave antenna, does this make the system more efficient than one that is closer to 3 meters?
Couple that with a coil that has taps every turn and would it be the best efficiency that we could expect for that frequency?
I'm just thinking that I can simplify things with the antenna construction down to where I buy a single larger diameter copper pipe. In one of the past threads it was determined that to have much impact on signal quality the antenna would need to be larger than 6 inch pipe, but I think a single length of something like 1 1/2 inch pipe might work better with the wind that we get here. I'm not that concerned about being able to adjust the antenna length for a different frequency.
Am I thinking correctly or way off course?
Realizing that part 15 AM is far from an ideal antenna length, I was wondering if making the antenna an even multiple of the wavelength would be of much help.
Physics shows that a short, linear, vertical monopole radiator for Part 15 AM would not work better for being exactly some small fractional wavelength in height (sorry). Radiation resistance should be maximized, because that is the parameter that determines how efficiently the antenna system will radiate (other things equal). Radiation resistance is strictly a function of the antenna length, and does not peak or have any other special value if that length is exactly a small fraction of a wavelength. So the longer the radiator, the better, up to the 3-m legal length.
Radiator width determines its reactance at that frequency, and the rate at which that reactance changes with frequency. System efficiency when using average-to-poor r-f grounds typical for Part 15 AM can be a bit higher if the radiator reactance is minimized , because that means the loading coil reactance needed to resonate the radiator can be reduced, which might reduce coil loss. But lower coil loss might also affect system r-f bandwidth as I posted earlier, so that needs to be considered.
A 3-m radiator is the best length for all Part 15 AM frequencies (per Part 15.219), and an OD of 1/2" should give acceptable results as far as keeping the loading coil reactance at a tolerable value (YMMV). But the small trims in the length of the "3-m" radiator suggested by Phil_B would make no practical difference in received field strengths, as long as the antenna system was resonant in all cases, and other parameters were equal.
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I once experimented with a coil that had taps every two turns like the original, but at the top I tapped the top two turns every 45 degrees for a total of 16 taps.....
Here's another thought that maybe you guys can think about. Instead of a bunch of taps on the top two turns for fine tuning, maybe the top two turns could be wound with bare wire spaced sufficiently to keep from shorting and to allow the antenna wire to be soldered.
This would be a lot easier than a bunch of taps and would provide continuous fine tuning without the pipe length adjustment.
Seems that we are almost to the point of building the coil so that there is space between the windings, and then compressing or expanding the coils to provide the adjustment. Could you do this on just a part of the coil? Wind the first half close, and then the area where there would be taps you could wind loose and just move the coils to fine tune the system. Again though we are probably not going to get much in the way of gains. And it may or may not make the construction and adjustment more complex to the point of why bother? But it is something to think about as an alternate to bringing a soldering iron up the tower.
As far as I know having a two part coil would be a very attractive solution. You're basically describing inductors in series, where one inductor is fixed in value and the other can be adjusted by changing it's physical properties.
Passive components like inductors are subject to basic rules when combined. Inductors in series add inductance.
There are also impacts on current and Q, but we're talking about low currents and very small adjustments so those impacts could be minimal.
If I recall the days of my youth ripping apart old AM radio sets there were lots of examples of two part and even multi-use inductors as the manufacturing became more sophisticated.
As far as the gains from this type of effort, to my way of thinking there won't be any theoretical gain in calculated power delivered to the antenna BUT you will have a better chance of finding that magical point in the very narrow bandwidth of these antennas where it is as resonant as possible.
In my experience, it's finding the combination of adjustments that gets closest to resonance that really makes the difference in range between a few yards and a few blocks.
I think this is where the misunderstanding sometimes occurs in trying to connect the theoretical to the real experiences of broadcasters lucky enough to have tuned a system to resonance. When I have hit that golden adjustment the range increases exponentially, just like hitting a harmonic on a guitar string.
It's cool!
Pretty good suggestions!
Experimental broadcasting for a better tomorrow!
Ok, this is probably a very dumb question.. But I'll ask it anyway..
How can it be a case of "the length doesn't matter, just make it as long as possible within the 3 meter total limit" and the length of the radiator having to be tuned for best resonance both be true?
Am I missing something here, or are there two (or more) schools of thought on this matter?
Daniel
Rattan wrote: How can it be a case of "the length doesn't matter, just make it as long as possible within the 3 meter total limit" and the length of the radiator having to be tuned for best resonance both be true?
Am I missing something here, or are there two (or more) schools of thought on this matter?
No, you're not missing anything, this is just a function of how complex this issue really is.
On the one hand you can use an antenna of fixed length (say 3 meters) and fine tune using an inductor and/or capacitor. That's the way this antenna works
On the other hand you can have an antenna with a fixed air coil inductor and tune it by varying the length of the antenna radiator, like this antenna
You can also use an antenna of fixed length and fine tune the air coil inductor using a piece of aluminum like 12vman did here
Or, you can use an iron powder bar in a small inductor to tune like the Talking House
This is why some of us are less inclined to accept the concept of a set of hard and fast rules for antenna construction. There is only one school of thought, because the RF obeys the laws of physics - but there are several ways to get to school 🙂
Experimental broadcasting for a better tomorrow!
What If... instead of using a single wire or element for the antenna once you get past the coil.
Why not have a vertical loop (or a couple of loops) within a 8 foot PVC pipe....and ...in theory as long as the final assembly is under 3 meters total ( in height) ..this should be legal and perhaps perform better than a single element or a helical antenna.
Your thoughts?
Why not have a vertical loop (or a couple of loops) within a 8 foot PVC pipe....and ...in theory as long as the final assembly is under 3 meters total ( in height) ..this should be legal and perhaps perform better than a single element or a helical antenna.
It could be true - this configuration might have been overlooked by antenna engineering experts in the past.
So this is an exciting concept, and from what we read here we all should be positive, encouraging, and avoid "pooh-pooh"-ing it.
With that in mind, let us all hope that you or someone will expend the required time, effort and cost to experiment with this configuration, and report back to us with defensible results. It might even be useful in that process to study the research that has already been done on this subject, and included in antenna engineering textbooks.
Could this configuration provide a great improvement in the coverage of "legal" Part 15 AM stations?
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