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in the last ten years we have seen many part 15.219(b) am transmitter designs. some with rather unique arrangements to squeeze every bit of efficiency from the final rf stage and antenna. i was thinking then it hit me. tubes are high voltage low amperage (hi impedance) just like that shortened whip used under part 15 am. i am thinking along the lines of a tube final stage with a solid state hi stability pll oscillator and modulator stage and a very large cage wound coil loading a 3 meter whip over 32 x 20ft copper radials featuring a balanced input and be capable of 130% positive modulation.
i am wondering how this idea might compare to fully solid state designs.
i would be interested in veteran engineers thoughts on this idea.
i am just a experimenter and what i know about RF circuit design can fit on the head of a pin i have a very basic grasp of transmitter design so i am interested in the thoughts of someone with more experience in rf design than me.
Rev. Robert P. Chrysafis
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Total posts : 45366Robert, your thoughts on this are reasonable but the problem with feeding power into a short antenna is that the antenna is almost pure capacitive reactance. Reactance does not dissipate power so even if the transmitter matched the 3000 ohms or so for the antenna no real power would be transferred to this reactance.
The purpose of the base loading coil is to insert inductive reactance equal to the capacitive reactance of the antenna and these being in series will cancel leaving only the resistive component which does dissipate the power.
Neil
Total posts : 45366will the higher impedance output of the tube final require less of a coil to appropriately match the capacitive load or will it not have any more total gain over the low impedance solid state final with more resistance?
Total posts : 45366Ken Cornell, W2IMB, wrote a series of books for
Radio Experimenters about Part 15 on the AM BCB
and the LW Part 15 band. He is no longer with us
and the books are out of print.There were ten editions of the book (I think) that he
sold from the early 1970s to the late 1980s (?).Ken had some tube transmitter designs that used the
6V6 as a final. He noted that the 6V6 ran very efficiently
using 20 volts on the plate. (I’m not sure what that really
meant – I suppose it just referred to the efficiency of the
tube.) The rig ran 100 mW into the final amp. The antenna
was 10 feet. (I think the rule was different in those days –
it was ten feet, and not 3 meters.)Although the transmitters used modulation transformers and
could run AM, Ken and anther experimenter 11 miles away
made successful contacts on cw. Because Part 15 above 1600
kHz was not legal in those days, they had to go below 1600 kHz.The frequencies in use were 1575, 1585, and 1595 kHz. And
again, they were successful over an 11 mile path.Although I have some of Ken’s circuits on paper at home, I do not
have the tube designs. However, I have always been intrigued by
that 6V6 final, and how well he said it worked.Is this relevant to what you guys are talking about? I’m not sure.
Best Wishes,
Bruce, DOGRADIO Studio 2
Total posts : 45366My leap into this discussion is between a guess and an opinion, but the subject is interesting.
I just looked up the 6V6, since it was mentioned, in the RCA Receiving Tube Manual, and its plate runs between 50k-ohms and 80k-ohms, which is way higher than the 3k-ohm antenna impedance mentioned by Radio8z.
When power is being transferred in a circuit, whether its a transmitter/antenna or an amplifier/loudspeaker, the most efficient transfer requires a true impedance match.
Based on my reasoning, a tube circuit would not have any advantage over solid-state, but the matching between plate and antenna would be a different batch of parts.
Tubes really get fun when you can crank up the plate voltage and get orange and blue glows from the plate.
Total posts : 45366will the higher impedance output of the tube final require less of a coil to appropriately match the capacitive load….
No because the impedance seen at the feedpoint of an antenna system is determined by the antenna system characteristics. A given antenna has a reactance determined by the antenna and this fixes the inductance of the coil needed to have resonance. At resonance, the impedance seen at the feedpoint is resistive with this resistance being determined by the antenna losses and radiation resistance. Many transmitters have internal loading coils or tuning networks which provide the matching but the best way to look at this is that the matching network is part of the antenna system.
…its plate runs between 50k-ohms and 80k-ohms, which is way higher than the 3k-ohm antenna impedance…
Yes, and the impedance of the load needs to be transformed up to operate with the high source impedance. This is why tube audio amps (and some transistor amps) use transformers. At RF, this transformation can be achieved by the matching network.
…the most efficient transfer requires a true impedance match.
Acutally this is not the case. The “Maximum Power Transfer Theorem” states that maximum power transfer from source to load occurs when the source impedance equals the conjugate of the load impedance, but under this condition half the power is lost in the source and the efficiency is 50%. The best way to drive a load is to have a source impedance which is very much less than the load impedance. This is necessary to get high efficiency.
Based on my reasoning, a tube circuit would not have any advantage over solid-state, but the matching between plate and antenna would be a different batch of parts.
Exactly right. Tube circuits are high voltage low current and solid state circuits are low voltage and high current. For a given power delivered to the load the volts and amps at the load are the same for both if properly matched.
Hope this helps.
Neil
Total posts : 45366ok. guess it’s back to thinking again about how to improve efficiency of the overall system
Total posts : 45366“how to improve efficiency of the overall system”
A MAJOR improvement in efficiency, even with an old school TX barely reaching 50 percent efficiency, is a LONGER antenna allowance.
It don’t matter if a 100 percent efficient transmitter was around, the major attenuator is the antenna and its INHERENT deficiencies galore.
So your right kc8, how to improve the efficiency of the overall system with inherently inefficient limits.
Maybe when the next element is discovered and added to the periodic table we will get our answer.
RFB
Total posts : 45366the big way is to maximize final efficiency and minimize antenna losses. we just have to find that magic combination that gives maximum erp. it can be done and i’m sure it can be done way better than whats presently available.
Total posts : 45366What else can be done except finding that miracle transmitter with 100 percent efficiency and miracle antenna with no losses at a mere 3 meter length?
I think the best approach (antenna wise) has already been discovered and marketed…the Isotron.
Problem is that it follows the industry standard 50 ohm impedance, and all these low power TX’s, including the AM5K, does not follow the industry standard of 50 ohms loading.
Then of course there is the ground system, available real estate to lay down a good ground system, ground conductivity and clearance around the antenna system.
If you have all the right parts and pieces the puzzle will come together just fine. But for those who are missing those perfect pieces of the puzzle, well there is not much else they can do to maximize things when limited by other things.
RFB
Total posts : 45366sstrans can be jumpered to 50’s ohms. just getting the sstran to tune to the full class e mode is a real bitch for an experienced electronics tinkerer let alone a novice, but it still is a pretty decent well built transmitter other issues aside.
Total posts : 45366Well as I pointed out, even with a 100 percent efficient transmitter, there still is the limited length antenna system that puts a pretty big road block in the way of any real sense of improvement.
The answer is not in increased efficiency of the transmitter, it is the antenna system and its ridiculous length limit that will forever keep all Part 15 AM stations regardless of what TX they are running, from reaching that ultimate peak performance.
So the answer is getting the FCC to revise the rules and increase that pathetic antenna length to at least 20 feet.
And then, as I pointed out once before, the improved efficiencies obtained in these new transmitters will really shine…heck even with older transmitters no where near the efficiency of new ones, longer antenna length will let those older transmitters really shine!
This is why I stick with CC operation. My signal travels along the power lines and there are power lines strung up everywhere, thus my signal gets everywhere I want it to go, and a heck of a lot further than my 3 meter backup stick, even if that backup stick was running an AM5K, my CC system would win hands down.
The AM5K unit is not the issue nor do I doubt it’s performance potential over other units or even its younger little 3K brother, but the fact remains that no matter what you use to generate that signal, your entire fast going endeavor stops at the 3 meter limit.
RFB
Total posts : 45366seems to me we can get 14 to 16 ft in most cases without the fcc blowing a gasket. they seem to not get their panties in a wad over mounting the tx on a 3-6ft mast then having a 3 meter base loaded whip and tx mounted on that mast from the various reports i hear. anything higher they tend to start getting a little prickly about it.
Total posts : 45366One time I put 20-feet of wire on a bamboo pole and stuck it vertically, but could detect NO improvement in coverage area.
Maybe some of the transmitters are so well geared for 3-meters that they go out of reach with longer lengths?
Total posts : 45366The Class E tx. I use will operate with a resistive load in the range of 20 to 110 ohms (I didn’t test it beyond these limits) with high efficiency so the antenna load is not a problem as long as it is tuned to resonance.
We need to find a way to cancel the antenna capacitive reactance without using the lossy coils.
Neil
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