adam@media-lab.MEDIA.MIT.EDU (Adam Glass) (07/29/90)
Could I use a high-frequency op-amp as a voltage follower to amplify the output of a little walkie talkie? I'd like to increase the thing to more than 100mW and get enough output out of it to boost the effective range up to a mile or two. Maybe up to 3 or 4 watts. Is this possible? Legal? If it isn't possible, how could I boost the signal to increase the range? Also, can the crystals just be desoldered and replaced with different frequencies as long as you get crystals of the same type? Or are other things in the circuit tuned to the crystal's frequency? Thanks. Adam
henry@zoo.toronto.edu (Henry Spencer) (07/29/90)
In article <3011@media-lab.MEDIA.MIT.EDU> adam@media-lab.MEDIA.MIT.EDU (Adam Glass) writes: >Could I use a high-frequency op-amp as a voltage follower to amplify >the output of a little walkie talkie? ... >Is this possible? Legal? ... Possible, probably. Legal, no. For one thing, there are severe limits on the power output of unlicensed radio transmitters. For another, such transmitters have to have FCC approval, and *any* private modification voids that approval. Only people with proper certification -- notably, radio amateurs -- can build or modify their own transmitters. There are a few specialized exceptions to this, e.g. for operations in the extreme ends of the spectrum or at very low power, but none that would cover what you propose, as far as I know. >Also, can the crystals just be desoldered and replaced with different >frequencies as long as you get crystals of the same type? Or are >other things in the circuit tuned to the crystal's frequency? In general, such mass-market products will be "single point" designs, optimized for operation under specific conditions. Also, changing the frequency is another illegal modification, and one with potentially very serious consequences. If you end up interfering with something like emergency services or aircraft navigation, you could kill someone. Even if you don't, the Feds have absolutely no sense of humor about such interference, and will jump on you with both feet. This is a possibility even if you don't switch frequencies, given that a homebrew amplifier may well generate harmonics you didn't really intend to emit. If you have the skills to do such a modification properly and safely, you can easily become a licensed radio amateur so you can do it legally. -- NFS: all the nice semantics of MSDOS, | Henry Spencer at U of Toronto Zoology and its performance and security too. | henry@zoo.toronto.edu utzoo!henry
davet@tsdiag.ccur.com (Dave Tiller N2KAU) (07/31/90)
In article <3011@media-lab.MEDIA.MIT.EDU> adam@media-lab.MEDIA.MIT.EDU (Adam Glass) writes:
-Could I use a high-frequency op-amp as a voltage follower to amplify
-the output of a little walkie talkie? I'd like to increase the thing
-to more than 100mW and get enough output out of it to boost the
-effective range up to a mile or two. Maybe up to 3 or 4 watts.
-
-Is this possible?
No. Unless you find a very high current op amp, you can't supply enough
current (remember it is power we're talking about here) to get 3-4 Watts.
Most op amps can only source a few mA.
-Legal?
Probably not. You didn't mention what band the walkie talkie is in, but if
it's anywhere but the Citizen's band, what you propose is _very illegal_.
Please don't add to the already widespread problem of illegal operators
in the non-CB bands. I spend enough time chasing after illegal operators
brazen enough to operate _inside_ the Hams bands, I don't want to have to
chase people in the cordless phone band as well. (flamestopper: I only
actively chase and tattle on people that infringe on my rights as either
a legal Amateur operator or cordless phone user. This does not include
inconsiderate _legal_ operators. Case in point: the commercial fishermen
on 10m FM 28.185)
-If it isn't possible, how could I boost the signal to increase the range?
If it's in the CB band, buy a better one. They do sell 4 Watt handhelds.
If it's a 49MHz version, either use better receivers or by a 4W CB one.
-Also, can the crystals just be desoldered and replaced with different
-frequencies as long as you get crystals of the same type? Or are
-other things in the circuit tuned to the crystal's frequency?
Sort of. If the crystals are reasonably close to the design frequency,
you shouldn't have too much trouble by just replacing them. (Just how far
you can go depends on the design of the radio and the frequency you're
dealing with.) If you want to go relatively far, you'll have to re-tune
the front end of the receiver and parts of the transmitter.
-
-Adam
--
David E. Tiller davet@tsdiag.ccur.com | Concurrent Computer Corp.
FAX: 201-870-5952 Ph: (201) 870-4119 (w) | 2 Crescent Place, M/S 117
UUCP: ucbvax!rutgers!petsd!tsdiag!davet | Oceanport NJ, 07757
ICBM: 40 16' 52" N 73 59' 00" W | N2KAU @ NN2Zdt@yenta.alb.nm.us (David B. Thomas) (08/01/90)
adam@media-lab.MEDIA.MIT.EDU (Adam Glass) writes: >Could I use a high-frequency op-amp as a voltage follower to amplify >the output of a little walkie talkie? I'd like to increase the thing >to more than 100mW and get enough output out of it to boost the >effective range up to a mile or two. Maybe up to 3 or 4 watts. >Is this possible? Legal? Yes, no. Fun and useful? Yes. The circuit you are looking for is available from a hobby mail order outfit: Panaxis Productions P.O. Box 130 Paradise, CA 95967-0130 (916) 534-0417 M-F 9am-6pm CA time They sell plans and kits for all sorts of things, including bugging and anti- bugging devices, pirate radio gear, radio transmitters and amplifiers of all sorts, etc. I have bought (but of course never used :-) several of their products. The plans are often sketchy, but the circuits always work as advertised, so far. The specific product you want is the "peak power booster". It's an ingenious walkie-talkie power amplifier. It switches your carrier on and off 8,000 times a second, providing 5 watt peaks, but averaging very low current (long battery life). The receiver is very unlikely to be able to reproduce the 8,000 Hz whine and will get only your voice, just as strong as if it were transmitted 5W continuous. I haven't built this one yet, but the plans look easy enough for a hardware hacker type. David
root@lando.la.locus.com (The Super User) (08/02/90)
In article <3011@media-lab.MEDIA.MIT.EDU> adam@media-lab.MEDIA.MIT.EDU (Adam Glass) writes: >Could I use a high-frequency op-amp as a voltage follower to amplify >the output of a little walkie talkie? I'd like to increase the thing >to more than 100mW and get enough output out of it to boost the >effective range up to a mile or two. Maybe up to 3 or 4 watts. > >Is this possible? Legal? If it isn't possible, how could I boost the >signal to increase the range? I'm curious why you'd want to use an IC where a single transistor is appropriate. There aren't may op amps, certainly no affordable op amps, that do what you want. There are lots of transistors, though, and they don't cost much. For the rest of this posting, I'll assume you are using a 49 Mhz FM walkie talkie. Normally for FM, a class-C amplifier is desirable, but the low output of the HT may require a class-A type amplifier. There are many choices of power transistors; you could use a plentifully available 2N3866 with the appropriate circuit. But, now that I've suggested how, DON'T do this. It is illegal and inconsiderate. There are many users of the 49Mhz allocation; if you crank up you power so you can have a 1 mile+ range, you'll also interefere with people's cordless phones and baby monitors for some distance. >Also, can the crystals just be desoldered and replaced with different >frequencies as long as you get crystals of the same type? Or are >other things in the circuit tuned to the crystal's frequency? Maybe. I've seen a lot of HTs for the 49 Mhz band that have only one crystal and a synthesizer. Changing the rock can certainly get you some new channels. In the case that a single channel is provided, you could change the crystal and probably not have to re-tune anything, since the HT is fairly broadband (at least across the narrow 49 Mhz allocation). What are you trying to accomplish? This may be the wrong approach....
markz@ssc.UUCP (Mark Zenier) (08/03/90)
In article <1694@yenta.alb.nm.us>, dt@yenta.alb.nm.us (David B. Thomas) writes: > adam@media-lab.MEDIA.MIT.EDU (Adam Glass) writes: > >Could I use a high-frequency op-amp as a voltage follower to amplify > >the output of a little walkie talkie? > > The specific product you want is the "peak power booster". It's an > ingenious walkie-talkie power amplifier. It switches your carrier on and > off 8,000 times a second, providing 5 watt peaks, but averaging very low > current (long battery life). The receiver is very unlikely to be able to > reproduce the 8,000 Hz whine and will get only your voice, just as strong > as if it were transmitted 5W continuous. This is very hazardous to your health. Since it would put out strong sidebands +/- 8 khz of your signal (and probably multiples of same) and wipe out at least the two adjacent channels if used on 27 Mhz CB, thus resulting in a crowd of pickup trucks showing up at your front door with irate occupants. markz@ssc.uucp
peter.saulesleja@f630.n250.z1.fidonet.org (peter saulesleja) (08/05/90)
> No. Unless you find a very high current op amp, you can't > supply enough > current (remember it is power we're talking about here) > to get 3-4 Watts. Why high current? There is very little current travelling through the antenna, so you'd require a higher voltage, wouldnt you? After all, P=VI... And air is a very good insulator. A high current would be impossible, at reasonable (below 1,000,000 Volts) voltages. -=Peter Saulesleja=- --- Opus-CBCS 1.13 * Origin: Nowhere(833-2814) "I hate origin lines!" (1:250/630.0)
logajan@ns.network.com (John Logajan) (08/08/90)
peter.saulesleja@f630.n250.z1.fidonet.org (peter saulesleja) writes: >Why high current? There is very little current travelling through >the antenna, so you'd require a higher voltage, wouldnt you? After >all, P=VI... And air is a very good insulator. A high current would >be impossible, at reasonable (below 1,000,000 Volts) voltages. The impedance of the antenna/radio wave interface is generally on the order of 50-300 ohms. Given this "resistance" you can come up with the voltages needed for a given power. For simplicity, to drive one watt into a 100 ohm load would require 10 volts. -- - John Logajan @ Network Systems; 7600 Boone Ave; Brooklyn Park, MN 55428 - logajan@ns.network.com, john@logajan.mn.org, 612-424-4888, Fax 424-2853
dana@lando.la.locus.com (Dana H. Myers) (08/08/90)
In article <1990Aug7.182544.7140@ns.network.com> logajan@ns.network.com (John Logajan) writes: >peter.saulesleja@f630.n250.z1.fidonet.org (peter saulesleja) writes: >>Why high current? There is very little current travelling through >>the antenna, so you'd require a higher voltage, wouldnt you? After >>all, P=VI... And air is a very good insulator. A high current would >>be impossible, at reasonable (below 1,000,000 Volts) voltages. > >The impedance of the antenna/radio wave interface is generally on the >order of 50-300 ohms. Given this "resistance" you can come up with >the voltages needed for a given power. For simplicity, to drive one >watt into a 100 ohm load would require 10 volts. More specifically, the formula is: P = VI and: I = V/R combine the two, you get: P = V^2 / R Of course, the RF output is usually expressed in RMS Watts, so the Peak-Peak output must be 1.414 (square root of 2) greater than the RMS voltage derived above, therefore you can work them around a little: V (Peak to Peak) = sqrt(2PR) V (RMS) = sqrt(PR) P (rms) = V (P-P) / 2R Enjoy. ***************************************************************** * Dana H. Myers KK6JQ | Views expressed here are * * (213) 337-5136 (ex WA6ZGB) | mine and do not necessarily * * dana@locus.com | reflect those of my employer * *****************************************************************
davet@tsdiag.ccur.com (Dave Tiller N2KAU) (08/08/90)
In article <1990Aug7.182544.7140@ns.network.com> logajan@ns.network.com (John Logajan) writes: -peter.saulesleja@f630.n250.z1.fidonet.org (peter saulesleja) writes: ->Why high current? There is very little current travelling through ->the antenna, so you'd require a higher voltage, wouldnt you? After ->all, P=VI... And air is a very good insulator. A high current would ->be impossible, at reasonable (below 1,000,000 Volts) voltages. - -The impedance of the antenna/radio wave interface is generally on the -order of 50-300 ohms. Given this "resistance" you can come up with -the voltages needed for a given power. For simplicity, to drive one -watt into a 100 ohm load would require 10 volts. - At 100 mA. Find an op-amp that can deliver 100 mA at a 10 V p-p at 27 MHz. Mercy, what a slew rate! As for using a high voltage, remember that this thing is battery supplied, and that the antenna is waving around at people. I wouldn't want to see high RF voltage flying around my head from an exposed antenna. The easiest method, if it were legal, would be to use a final transistor current amp (common collector). There are lots of transistors that'll handle 27 MHz just fine. -- David E. Tiller davet@tsdiag.ccur.com | Concurrent Computer Corp. FAX: 201-870-5952 Ph: (201) 870-4119 (w) | 2 Crescent Place, M/S 117 UUCP: ucbvax!rutgers!petsd!tsdiag!davet | Oceanport NJ, 07757 ICBM: 40 16' 52" N 73 59' 00" W | N2KAU @ NN2Z
myers@hpfcdj.HP.COM (Bob Myers) (08/09/90)
>Why high current? There is very little current travelling through >the antenna, so you'd require a higher voltage, wouldnt you? After >all, P=VI... And air is a very good insulator. A high current would >be impossible, at reasonable (below 1,000,000 Volts) voltages. Tain't that simple; there are in fact high *currents* present in various parts of an antenna (exactly where depends on the antenna design). For example, in the simple dipole antenna, the feedpoint (center) is an area of high current, while the ends are low (supposedly zero) current and high voltage. It wouldn't work, otherwise. (The dipole can best be understood by thinking of it as a quarter-wavelength, open-ended length of transmission line, which has been "opened up.") The feedpoint impedance of a properly tuned dipole is about 73 ohms, so you can figure the feedpoint current based on this value and the power. If we assume 4 watts, then there's about 230 mA at the feedpoint - which is pretty "high current" for an op-amp, assuming you can find one that'll work at the frequency in question. Bob Myers KC0EW HP Graphics Tech. Div.| Opinions expressed here are not Ft. Collins, Colorado | those of my employer or any other myers@fc.hp.com | sentient life-form on this planet.
usenet@ucbvax.BERKELEY.EDU (USENET News Administration) (08/10/90)
> > No. Unless you find a very high current op amp, you can't > > supply enough > > current (remember it is power we're talking about here) > > to get 3-4 Watts. > >Why high current? There is very little current travelling through >the antenna, so you'd require a higher voltage, wouldnt you? After >all, P=VI... And air is a very good insulator. A high current would >be impossible, at reasonable (below 1,000,000 Volts) voltages. > > > -=Peter Saulesleja=- > > >--- Opus-CBCS 1.13 > * Origin: Nowhere(833-2814) "I hate origin lines!" (1:250/630.0) From: hull@janus.Berkeley.EDU (Christopher Hull) Path: janus.Berkeley.EDU!hull An antenna is hooked up so that it looks like a short circuit at D.C. and is usually 75-300 ohms at the RF frequency. You need both higher voltage and higher current to put out large amounts of power. Tranformers can be used to convert a high voltage and low current to a high current and low voltage or vice-versa. A standard 9v Cell will not delver enough power to drive more the 100mw for a reasonable amount of time. Hence, most powerfull C.B.'s use banks of 8-12 AA cells to derive the necessary power output over a sustained time. Design of a power amp also requires transistors with enough power handling capability. The design requieres some knowlidge of electronics beyond Ohm's law. Chris Hull hull@Janus.berkeley.edu
dana@lando.la.locus.com (Dana H. Myers) (08/11/90)
In article <1063@tsdiag.ccur.com> davet@tsdiag.ccur.com (Dave Tiller N2KAU) writes: [ talking about a power amp for a walkie-talkie and using an op-amp ] > The easiest method, if it were legal, would be to use a final >transistor current amp (common collector). There are lots of transistors >that'll handle 27 MHz just fine. Common collector? Hmm, most of the RF amplifiers use NPN transistors in a common emitter configuration. At 27 Mhz, you could even use a cheap power MOSFET like the IRF511; I built a 10m amp using one of these and got something like 13 dB+ of power gain at 29.0 Mhz (15W out). Of course, the MOSFET amplifier was common source, with about 50mA of idle current. What was funny, for that project I built an RF-sensed T/R switch, and ended up using the same active device as the power amplifier to drive a relay... Once again, however, please DO NOT build your own power booster for a 27Mhz or 49Mhz handie talkie. ***************************************************************** * Dana H. Myers KK6JQ | Views expressed here are * * (213) 337-5136 (ex WA6ZGB) | mine and do not necessarily * * dana@locus.com | reflect those of my employer * *****************************************************************
dana@lando.la.locus.com (Dana H. Myers) (08/11/90)
In article <38070@ucbvax.BERKELEY.EDU> hull@janus.Berkeley.EDU) writes: > >An antenna is hooked up so that it looks like a short circuit at D.C. and >is usually 75-300 ohms at the RF frequency. Not necessarily, it depends on the matching networks in use. Some antennas have 'baluns' or some form of shunt matching network installed. For instance, a quarter-wave ground plane which has the radiating element perpendicular to the ground plane will not directly match a 50 ohm line. A match may be achieved by placing some 'shunt reactance' from the radiating element to ground, this could be an inductor or capacitor. An inductor would appear as a DC short to ground. (IMHO, the best way to match a quarter-wave is to droop the radials) >Design of a power amp also requires transistors with enough power handling >capability. The design requieres some knowlidge of electronics beyond >Ohm's law. I'll second this motion. An improperly built RF power amp can be a real nuisance. I've seen some real hacks out there. For the record, I've built several amps from scratch, sometimes just to experiment. ***************************************************************** * Dana H. Myers KK6JQ | Views expressed here are * * (213) 337-5136 (ex WA6ZGB) | mine and do not necessarily * * dana@locus.com | reflect those of my employer * *****************************************************************
myers@hpfcdj.HP.COM (Bob Myers) (08/14/90)
>An antenna is hooked up so that it looks like a short circuit at D.C.......
Not always; for example, a normal half-wave dipole is an OPEN to DC. The
DC resistance depends completely on the antenna design in question, and has
very little to do with the RF feedpoint impedance.
(Some designs which would show a "short at DC" would be a vertical with a coil
between feedpoint and ground, or a loop antenna, or various feeds of "normally
open" antenna types, such as a delta match on a dipole. One other which
wouldn't would be the "normal" log-periodic, such as is used for TV reception.)
Bob Myers KC0EW HP Graphics Tech. Div.| Opinions expressed here are not
Ft. Collins, Colorado | those of my employer or any other
myers@fc.hp.com | sentient life-form on this planet.