- Posts
Hello all,
In a recent post here I posted:
[quote]I would look instead at the efficiency of the part15 AM transmitters. Theoretically, a Class C amplifier can have 100% efficiency but practically it is closer to 70% based on my experience building VHF amplifiers. I measured the efficiency of my Ramsey AM-25 in a bench test with several different load resistances. The maximum efficiency was 31% with a load of 24.3 ohms. With a legal input of 100 mW. this gives an output to feed the antenna of 31 mW. (though the actual powers produced in my test were different). If this circuit could be improved to a 70% efficiency, it could increase the range by 40% legally. [/quote]
Hello all,
In a recent post here I posted:
[quote]I would look instead at the efficiency of the part15 AM transmitters. Theoretically, a Class C amplifier can have 100% efficiency but practically it is closer to 70% based on my experience building VHF amplifiers. I measured the efficiency of my Ramsey AM-25 in a bench test with several different load resistances. The maximum efficiency was 31% with a load of 24.3 ohms. With a legal input of 100 mW. this gives an output to feed the antenna of 31 mW. (though the actual powers produced in my test were different). If this circuit could be improved to a 70% efficiency, it could increase the range by 40% legally. [/quote]
At the time of this post I had no efficiency data for a transmitter other than the Ramsey AM25. I have, since this post, measured the efficiency of my SSTRAN AMT3000 and thus confirmed my opinion that the best gains in part15 range will be achieved by producing transmitters with efficiencies higher than I have seen.
I am, for now, keeping these data private, since I would hope that I am not the only one in the part 15 world who is doing some experiments. Take my word for it, your time is better spent on measuring and increasing transmitter efficiency than on installing more and longer radials under your antenna.
Neil
Total posts : 45366Isn’t the efficiency directly related to the coupling (match) between the antenna and transmitter? Or are you saying that there is a huge waste in power going through the amplifier circuits? I assume you are testing into a dummy load and that your SWR is extremely low.
I would be very interested in any mods made to the AMT3000 that would allow me to remain legal, and get more range.
Total posts : 45366Greg,
By definition the efficiency of an AM transmitter is the AC power delivered to the load divided by the DC power into the final amplifier x 100%. The FCC rules limit the DC power input to 100 mW. Assuming that this is a fixed number, then the gains can be had by increasing the efficiency of the amplifier. We could get up to 100 mW. RF power out if we can achieve 100% efficiency. For a transmitter with an efficiency of 30%, over two thirds of the input power is wasted as heat, and if we could get the efficiency up to around 70% we will more than double the power to the antenna. 70% is not unrealistic for a Class C amplifier.
I do not have any specific mods for any transmitter since I have not given this much thought, but I can give some general information. As I or others think about this we can post more information later.
For the test load I used a 28 ohm 2W resistor soldered to the center and shield of a 6 inch length of 50 ohm coax cable with a BNC connector on the end. The coax is too short to cause SWR problems at this frequency. I used 28 ohms because it is what I had in stock and because this is close to the predicted resistance of a properly tuned 3 m antenna with a loading coil and buried ground radials. The resistance and coax length are similar to what I would expect in a situation where the transmitter is mounted at the base of the antenna.
The efficiency is not directly related to the antenna coupling within limits. The power out is and the power in is. If by changing the match if you increase the power out then the power in will also increase. Since efficiency is the quotient of these two powers, it will not change much…just a bit due to possible nonlinear correlation between the power out and power in. I am not saying that a proper match is not important, and it is a factor in efficiency and is key to delivering the power to the load, but there are other things affecting efficiency.
After I ponder this for a bit, I will post more.
Neil
Total posts : 45366Thanks Neil. The reason this is of so much interest to me is I’m still waiting to buy an AMT3000 and it would be great to have everything needed to do a mod when it’s time to build it.
We finally have our stream up, so now it’s just about time to tackle the next task.
Total posts : 45366I did not nor do I intend to raise false hopes about fixes to existing designs which will improve efficiency.
Concerning the SSTRAN AMT3000, this apparently was designed to provide optimum performance with modulation linearity which it does almost to a laboratory standard. I fear any changes to the circuit will upset this performance and I recommend against this. Since many posters have reported good range with this unit using their recommended coil loaded antenna, I would not fear investing in this (as I have). It may not be optimum in terms of efficiency but it is effective.
My hope is that some experimenters will take up this subject and develop high efficiency AM circuits. I am not willing to do this myself, but will be willing to advise.
Sorry if I implied that there is a simple fix.
Neil
Total posts : 45366If it was simple, it would have already been done. I have a feeling that it will turn out to be an extremely complex mod, if even possible. But hopefully smarter minds than me will take up the challenge. Getting our stream going is really going to put a charge back in our students, and I’d like to carry that to another level, so I need to get things approved and purchased for the AM broadcast.
Total posts : 45366IMO..
The antenna system is a tank circuit that need to be tuned to the lowest resistance to the final amplifier for maxium output..I’ve assembled an SStran and played around with it on a service monitor. The input impedence is at 50 ohms. I could only get 28 milliwatts out of it and thought something was wrong with the output transistors. I replaced them and checked again. I didn’t gain anything..
I will trust that the designers of the circuits took a lot of variables into consideration, Hence the different reactance settings in the output section. The current/voltage peaks are only going to be achieved when some kind of resonance is found in the antenna system. We all know a perfect resonance will be difficult with the antenna restrictions and one can only make the best with what one has. I don’t believe any mods to the output circuit will make much difference. If one thing is changed, It just imbalances something else..
I’m a perfectionist by nature most of the time and I about drove myself nuts trying to make my SStran get the best range possible. I ran 1000’s of feet of wire for the ground, Built the base loaded antenna, Tried different heights above the ground area, Laid chicken wire around the base, Ect.. All this did was cause an imbalanced antenna system. (More ground area than antenna) Of course, The coil is adjustable to help find resonance but the coil is also lossy by nature. I adjusted my antenna system via field strength. I finally got fed up and went back to the basics..
I took a plastic work box that would hold the xmtr and mounted two 108″ whips via stud mounts to the box. (One pointing up, The other pointing down) Connected the antenna lead from the xmtr to the whip pointing up and the ground of the circuit to the one pointing down. I used a 10′ PVC pipe for the mast. I had no ground connection to any “Built” grounding area. I RF choked the audio/power leads in several places to avoid any coupling for a counterpoise ground as much as possible. This provided a dipole with two elements at the same length. Tuning by the instructions when using the provided “Wire” antenna to maxium gave me about the same range! This set up isn’t legal but it proved a point to me..
Fluctuations of voltage and current on the antenna is the important factor. The goal is to get these peaks as high as possible. Keeping things as balanced as possible makes finding the resonant point easier. If everything is beating in harmony as best as possible, That’s all one can do to get the max from the xmtr..
The output impedence is tough to figure. I believe the designers figured the wire antenna to be somewhere around 300 ohms? (I could be wrong) I’m just going on my understanding of an open dipole. Adjustments were designed in the xmtr to work around these perimeters. Easy set up, Easy tuning, Same results of months of messing with the ground issue..
Location has a bunch to do with range. Living trees will eat your signal up quick!
Total posts : 4536612vman,
Thanks for sharing the information. Though not as extensive as you, I have tried a few antennas and also noted no big difference in range.
I also have measured the SSTRAN output power and it is dramatically less than you reported. I used a different load R but I would like to replicate your results. How was the unit configured, for the wire ant. or the coil loaded antenna? Did you adjust the tuning cap for max output? My results may differ from yours since I made no attempt to “tune” the output and the unit is configured for the base loaded antenna.
The following does not imply that the SSTRAN is operating Class C and I am not sure that it is, but I have some general comments about transmitters. In the tube days, these amps. had tuned tanks where resonance was adjusted by tuning a cap for a dip in plate current. Then the drive and output network were adjusted to give the desired input power. I wonder if a tuned tank with an adjustable matching network (pi, L, or tee) would result in higher efficiency. I can understand that designers would prefer to avoid user accessable adjustments but I wonder if this would help.
A well understood characteristic of tube amps. is if operated off resonance the output will drop and the finals will overheat. This implies to me that the tuned tank is necessary for efficiency. Since solid state devices are current source devices as are tubes, it seems the same would apply to transistor amps.
Any thoughts on this?
Neil
Total posts : 45366from the SStran web site
[quote] Power input to the final RF stage is 100 milliwatts, the maximum allowed by the FCC for Part 15 operation without a license. The RF output and modulator stages use a unique and precisely engineered 5-transistor configuration that results in low distortion modulation right up 100%. [b]A tunable pi-network [/b]matches the RF output to the 118″ antenna (length limited by FCC regulations) without the need for any additional loading coils. Harmonic radiation is highly attenuated by the antenna tuning network.[/quote]So it looks like it already has a tuning network to optimize the output
Total posts : 45366Greg,
Without any data regarding efficiency or field strength I don’t know what to make of the tunable pi-network’s effectiveness. As an owner of a SSTRAN unit, I know I can tune the network for a peak voltage applied to the antenna but a peak with a 25% efficiency is not as effective as a peak with a 70% efficiency. If 12vman’s numbers are correct then I think we are a long way from optimum.
So, this may, as our math friends say “produce a local maximum” but it may not be a “global” maximum. In other words, it may produce the most effective use of an inefficient transmitter but it is, in my opinion, far from optimum with regard to what I think is possible.
I do not intend this to be a criticism of the SSTRAN unit. What I want to do is to shift the thinking away from antennas toward circuit design which I believe, through gains in efficiency, will provide the most benefit in the part 15 situation. Perhaps this has already been done, but I am not aware of it
As 12vman related, we seem to have approached the maximum performance from 3 m antennas and I am suggesting that we now look at the transmitter efficiency as the next frontier.
Neil
Total posts : 45366Could the difference be that yours has been modified for the external antenna, and 12vman’s has not been modified?
Since the antenna is a voltage fed device, would it make any sense to increase the operating voltage of the final RF stage? To meet FCC it would need a current decrease, but as long as we stay within the 100mw input power we’re OK right?
Are there any high effeciency transistors that could be substituted for the stock units?
As you can see I’m just kind of guessing right now.
I’m going to check with my boss Monday to see if I can order one of these yet. Now is the time for “fiddling” with it.
Total posts : 45366Greg,
You asked some reasonable questions for which I will attempt answers.
[quote]Could the difference be that yours has been modified for the external antenna, and 12vman’s has not been modified?[/quote]
Yes, and I asked that. I’ll have to wait for 12vman to reply.
[quote]would it make any sense to increase the operating voltage of the final RF stage?[/quote]
No. My unit runs at 89 to 93 mW. input depending on the load and an increase of a few milliwatts output power that may result won’t be noticable. This circuit was carefully designed to operate as it is and I wouldn’t try to change it.
[quote]Are there any high effeciency transistors that could be substituted for the stock units?[/quote]
At these frequencies and power levels, I doubt it. Generally the operation of a well designed circuit depends primarily on the choice of biasing and passive components. If that were not the case, then it would be necessary to configure each built circuit for each individual transistor since the gain (beta) spread of small signal transistors can vary 4 to 1 and still be in specification. By making circuit gain and biasing (and efficiency ) independent of transistor gain (beta) we don’t have to be concerned about the beta spread.
Since the circuit operation depends on the passive components, there would be little or no change with a transistor substitution. Changing resistors will change the circuit operation but since the SSTRAN unit runs at near the legal limit with very clean modulation I don’t see this as an option since it is already optimized.
I recommend that you build and adjust the unit as instructed in the manual. If you don’t then you might have problems which would not be helpful in making a good first impression when you go on the air.
Though I discourage modifying this device I am glad that your are thinking about options and hope my answers are useful.
Neil
Total posts : 45366Just going from my (old guy’s) memory, AM signals can be amplified through a single Class A (full-cycle) amplifier, but around 20% efficiency is the best that you can get. Class B (half-cycle) or AB (between half and full-cycle, usually 5/8, maybe 3/4) amps can be used alone for SSB signals or paired in back-to-back (push-pull) configuration for AM. Two Class AB amps in push-pull is more common with AM, and maximum efficiency is somewhere above 40%, so efficiency in the 30s is fairly reasonable. In addition, amplifiers usually run below maximum efficiency because of the use of negative feedback to keep the amp from oscillating.
You can use a Class C amp on FM, but not AM or SSB, as the amp is not linear. Old Class C amps (followed by a tuned “tank” circuit of coil & capacitor) were called “flywheel” amps. The amp is driven only by the peaks of the input signal, as you would periodically pump the foot pedal to keep a grinding or potters wheel going. For the rest of every cycle, the wheel’s momentum, or the oscillation of the tank circuit, keeps things going.
Just my $0.02…
Total posts : 45366QUOTE:
*How was the unit configured, for the wire ant. or the coil loaded antenna? Did you adjust the tuning cap for max output?*Hi, Neil..
I used the configuration for the wire antenna with the whips. I swapped the cap back and forth for the different situations..I’ve found that tuning with the whips was easy and the ideal “15 volt peak” could be approached closer than any of the other set ups I’ve tried. I used a fiberglass step ladder to get to the unit but things still changed a little when I moved towards/away from the xmtr. I could see the field strength change also when going up and down the ladder. I just found the happy medium the best that I could..
The power/audio leads will make a difference. They want to be counterpoise grounds and will sure give you headaches when tuning. I posted here a good while back about using an FM xmtr to get the audio to the AM xmtr. (The Best of Both Worlds) This avoids the use of the audio cables, Hence less counterpoise ground to worry about. I choked the crap out of the power cable which did help..
My goal is to set up a self contained solar powered unit to avoid all wiring leaving the xmtr. What I need then is a 25′ “Diddle Stick” to tune the cap.. LOL
I believe when I did testing with the monitor the xmtr was configured for the base loaded antenna. I never tried it with the other cap in circuit..
Total posts : 45366tregonsee,
I appreciate your comments and what you said about the various classes of amplifiers. Your characterization of class A, B, AB agrees with my understanding and they won’t achieve the efficiency that I think is possible with class C.
To expand on your comments, one can use a class C amp. for AM as long as the modulation is applied after the RF amplification. As you probably know this is called high level modulation and it is easily practical at the 100 mW. level requiring only 50 mW. of audio power. Maybe we should be more careful about calling them “amplifiers” and use the term “transmitters” instead since “amplifier” means make the signal bigger. As you expressed, a class C amplifier cannot be used to make an AM modulated signal bigger.
I have designed and built VHF (146 MHz.) class C amplifiers in the 5 to 40 watt range for FM which yielded efficiencies around 70%. At VHF this required a lot more care regarding selection of transistors and physical layout than I would expect at AM frequencies.
Regarding neutralization (negative feedback) this does reduce the gain but not the efficiency since in response to a lessened output signal the DC input power is also decreased.
Do you agree with me that a high level modulated class C AM transmitter could provide the efficiency needed to boost the output of part15 AM transmitters from 20 or 30 mW. or so to 70 mW. legally?
Thanks for your input on this.
Neil
- You must be logged in to reply to this topic.
