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Can I use wirewound resistors to build an HF attenuator?
I need to lose about 5 dB from a 15 Watt exciter. Thanks.
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Can I use wirewound resistors to build an HF attenuator?
"dave" wrote in message m... I need to lose about 5 dB from a 15 Watt exciter. Thanks. Simple answer is no. While you can build one with wirewound resistors , the normal formulars will not usually work at RF and you will have a lot of inductance to deal with. Even the socalled non-inductive resistors are only so at the audio frequencies. |
Can I use wirewound resistors to build an HF attenuator?
Ralph Mowery wrote:
"dave" wrote in message m... I need to lose about 5 dB from a 15 Watt exciter. Thanks. Simple answer is no. While you can build one with wirewound resistors , the normal formulars will not usually work at RF and you will have a lot of inductance to deal with. Even the socalled non-inductive resistors are only so at the audio frequencies. Can the inductive reactance be cancelled with capacitors? I'm mainly concerned with 40, 30 and 20 meters. |
Can I use wirewound resistors to build an HF attenuator?
I need to lose about 5 dB from a 15 Watt exciter. Thanks.
Simple answer is no. While you can build one with wirewound resistors , the normal formulars will not usually work at RF and you will have a lot of inductance to deal with. Even the socalled non-inductive resistors are only so at the audio frequencies. Can the inductive reactance be cancelled with capacitors? I'm mainly concerned with 40, 30 and 20 meters. The reactance will change with frequency. Do you want to be fiddling with antenna tuners? There is a whole lot of stuff you need to know when designing and building. A good place to start is with an ARRL Handbook. They can often be found in public libraries, but they are an excellent reference on the subjects you are dealing with. I own several because even the older information is useful. |
Can I use wirewound resistors to build an HF attenuator?
In message , dave
writes Ralph Mowery wrote: "dave" wrote in message news:0vqdnaEyq4zYAhfXnZ2dnUVZ_hJi4p2d@earthlink. com... I need to lose about 5 dB from a 15 Watt exciter. Thanks. Simple answer is no. While you can build one with wirewound resistors , the normal formulars will not usually work at RF and you will have a lot of inductance to deal with. Even the socalled non-inductive resistors are only so at the audio frequencies. Can the inductive reactance be cancelled with capacitors? I'm mainly concerned with 40, 30 and 20 meters. If your highest frequency is only 14MHz, and you're not trying to obtain a sooper-dooper perfect match for your exciter, then I'd say that you would certainly get away with using wirewound resistors. To minimise the effects of the inductance, you could try mounting the resistors 'hard down' against a ground plane. You might need a sheet of some insulating material to avoid any danger of shorting to it. And presumably, to get rid of 5dB (nearly 3/4 of your original 15W) you intend to use paralleled-up resistors. This in itself will help minimise the inductance. -- Ian |
Can I use wirewound resistors to build an HF attenuator?
In rec.radio.amateur.equipment dave wrote:
Ralph Mowery wrote: "dave" wrote in message m... I need to lose about 5 dB from a 15 Watt exciter. Thanks. Simple answer is no. While you can build one with wirewound resistors , the normal formulars will not usually work at RF and you will have a lot of inductance to deal with. Even the socalled non-inductive resistors are only so at the audio frequencies. Can the inductive reactance be cancelled with capacitors? I'm mainly concerned with 40, 30 and 20 meters. It becomes too frequency dependant to bother with. Use a T attenuator. See: http://chemandy.com/calculators/t-at...calculator.htm You will likely have to use series or parallel combinations of resistors to get the values required anyway. You can also play with using two attnuator sectoions to see if it makes the resistor values (and wattage required) simpler. -- Jim Pennino Remove .spam.sux to reply. |
Can I use wirewound resistors to build an HF attenuator?
["Followup-To:" header set to rec.radio.amateur.equipment.]
On 2009-08-18, dave wrote: I need to lose about 5 dB from a 15 Watt exciter. Thanks. Do you really need to lose some watts? Or do you just need to reduce the voltage to whatever you are driving? Maybe a capacitive voltage divider would do what you need. Jim W6JVE -- jhhaynes at earthlink dot net |
Can I use wirewound resistors to build an HF attenuator?
Jim Haynes wrote:
["Followup-To:" header set to rec.radio.amateur.equipment.] On 2009-08-18, dave wrote: I need to lose about 5 dB from a 15 Watt exciter. Thanks. Do you really need to lose some watts? Or do you just need to reduce the voltage to whatever you are driving? Maybe a capacitive voltage divider would do what you need. Jim W6JVE It's a Tokyo Hy-Power HL-45B amp, designed to be driven with a Yaesu FT-817. I'm using a "20 Watt" Argonaut V, which isn't terribly linear at 5 Watts (closer to 3 is actually what's required by the THP). |
Can I use wirewound resistors to build an HF attenuator?
"Ian Jackson" wrote in message ... In message , dave writes Ralph Mowery wrote: "dave" wrote in message news:0vqdnaEyq4zYAhfXnZ2dnUVZ_hJi4p2d@earthlink. com... I need to lose about 5 dB from a 15 Watt exciter. Thanks. Simple answer is no. While you can build one with wirewound resistors , the normal formulars will not usually work at RF and you will have a lot of inductance to deal with. Even the socalled non-inductive resistors are only so at the audio frequencies. Can the inductive reactance be cancelled with capacitors? I'm mainly concerned with 40, 30 and 20 meters. If your highest frequency is only 14MHz, and you're not trying to obtain a sooper-dooper perfect match for your exciter, then I'd say that you would certainly get away with using wirewound resistors. To minimise the effects of the inductance, you could try mounting the resistors 'hard down' against a ground plane. You might need a sheet of some insulating material to avoid any danger of shorting to it. And presumably, to get rid of 5dB (nearly 3/4 of your original 15W) you intend to use paralleled-up resistors. This in itself will help minimise the inductance. -- Ian A bunch of chip resistors in parallel to ultimately make a T-atten. |
Can I use wirewound resistors to build an HF attenuator?
In message , JB
writes "Ian Jackson" wrote in message ... In message , dave writes Ralph Mowery wrote: "dave" wrote in message news:0vqdnaEyq4zYAhfXnZ2dnUVZ_hJi4p2d@earthlink. com... I need to lose about 5 dB from a 15 Watt exciter. Thanks. Simple answer is no. While you can build one with wirewound resistors , the normal formulars will not usually work at RF and you will have a lot of inductance to deal with. Even the socalled non-inductive resistors are only so at the audio frequencies. Can the inductive reactance be cancelled with capacitors? I'm mainly concerned with 40, 30 and 20 meters. If your highest frequency is only 14MHz, and you're not trying to obtain a sooper-dooper perfect match for your exciter, then I'd say that you would certainly get away with using wirewound resistors. To minimise the effects of the inductance, you could try mounting the resistors 'hard down' against a ground plane. You might need a sheet of some insulating material to avoid any danger of shorting to it. And presumably, to get rid of 5dB (nearly 3/4 of your original 15W) you intend to use paralleled-up resistors. This in itself will help minimise the inductance. -- Ian A bunch of chip resistors in parallel to ultimately make a T-atten. Yebbut.... 12W is a fair bit to dissipate with chip resistors (even a lot of them). However, the question was if you could use WW resistors, not 'how to do it properly'. I presume there was a reason why this was asked! After all, we ARE talking 'amateur' radio. -- Ian |
Can I use wirewound resistors to build an HF attenuator?
In rec.radio.amateur.equipment Ian Jackson wrote:
In message , JB writes "Ian Jackson" wrote in message ... In message , dave writes Ralph Mowery wrote: "dave" wrote in message news:0vqdnaEyq4zYAhfXnZ2dnUVZ_hJi4p2d@earthlink. com... I need to lose about 5 dB from a 15 Watt exciter. Thanks. Simple answer is no. While you can build one with wirewound resistors , the normal formulars will not usually work at RF and you will have a lot of inductance to deal with. Even the socalled non-inductive resistors are only so at the audio frequencies. Can the inductive reactance be cancelled with capacitors? I'm mainly concerned with 40, 30 and 20 meters. If your highest frequency is only 14MHz, and you're not trying to obtain a sooper-dooper perfect match for your exciter, then I'd say that you would certainly get away with using wirewound resistors. To minimise the effects of the inductance, you could try mounting the resistors 'hard down' against a ground plane. You might need a sheet of some insulating material to avoid any danger of shorting to it. And presumably, to get rid of 5dB (nearly 3/4 of your original 15W) you intend to use paralleled-up resistors. This in itself will help minimise the inductance. -- Ian A bunch of chip resistors in parallel to ultimately make a T-atten. Yebbut.... 12W is a fair bit to dissipate with chip resistors (even a lot of them). However, the question was if you could use WW resistors, not 'how to do it properly'. I presume there was a reason why this was asked! After all, we ARE talking 'amateur' radio. A 5db T attenuator with 15 W in has the maximum dissipation in the shunt resistor at about 5 W. The input series resistor dissipates about 4 W and the output series resistor dissipates a little over 1 W. I see no reason to go to wirewound resistors at these power levels. -- Jim Pennino Remove .spam.sux to reply. |
Can I use wirewound resistors to build an HF attenuator?
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Can I use wirewound resistors to build an HF attenuator?
Can the inductive reactance be cancelled with capacitors? I'm
mainly concerned with 40, 30 and 20 meters. If your highest frequency is only 14MHz, and you're not trying to obtain a sooper-dooper perfect match for your exciter, then I'd say that you would certainly get away with using wirewound resistors. To minimise the effects of the inductance, you could try mounting the resistors 'hard down' against a ground plane. You might need a sheet of some insulating material to avoid any danger of shorting to it. And presumably, to get rid of 5dB (nearly 3/4 of your original 15W) you intend to use paralleled-up resistors. This in itself will help minimise the inductance. -- Ian A bunch of chip resistors in parallel to ultimately make a T-atten. Yebbut.... 12W is a fair bit to dissipate with chip resistors (even a lot of them). However, the question was if you could use WW resistors, not 'how to do it properly'. I presume there was a reason why this was asked! After all, we ARE talking 'amateur' radio. A 5db T attenuator with 15 W in has the maximum dissipation in the shunt resistor at about 5 W. The input series resistor dissipates about 4 W and the output series resistor dissipates a little over 1 W. I see no reason to go to wirewound resistors at these power levels. Oh, I quite agree. I wouldn't really choose to use WW myself, but I think they would work in this application. It's just that I usually try first to answer the question as asked, and, if necessary, start embellishing things from then on! http://www.youtube.com/watch?v=k7vMvlRio5Y&feature=related -- Ian He obviously has a source of WW on hand. I hate to place an order when I find stuff in the junk box. So now the question is: Will it be a significant problem? If he really knew what the inductance values were, then it would be a simple matter to make sure it is insignificant or at least minimal compared to the resistive component. If the resultant SWR is low enough, the 5 dB return loss will help. After that, the question would be "How will the amp act with whatever input load is there? Some resistors have a few laser cut turns that reverse themselves to cancel the inductance. Some are a Carbon pack. A true WW is either made by several turns of resistance wire over carbon or ceramic for larger power ratings. Those have way too many turns. It was mentioned that these even have a significant inductance at audio frequencies. Now I ask you, is 5k inductive reactance significant on a 50 ohm resistor? You can always build one and see, if the exciter SWR is under 2:1, the amp input would see that too and you might be OK! The other part of this is: How much power gain is the amp if you need to kill 5 dB. Another thought is to incorporate the inductance of the resistors to design a bandpass filter. With many solid state exciters, some of the spurious outputs don't reduce when you turn down the power. http://www.repeater-builder.com/tech...ndbook-4th.pdf All might find it helpful. A more legible html version: http://mikeyancey.com/files/Other%20...ion/title.html All food for thought. |
Can I use wirewound resistors to build an HF attenuator?
JB wrote:
"Ian Jackson" wrote in message ... And presumably, to get rid of 5dB (nearly 3/4 of your original 15W) you intend to use paralleled-up resistors. This in itself will help minimise the inductance. -- Ian A bunch of chip resistors in parallel to ultimately make a T-atten. The whole point of wirewound is that I can make a T pad with 3 5 Watt resistors. They make "flange" attenuators but they cost a fortune in small quantities. |
Can I use wirewound resistors to build an HF attenuator?
Ian Jackson wrote:
In message , JB writes "Ian Jackson" wrote in message ... In message , dave writes Ralph Mowery wrote: "dave" wrote in message news:0vqdnaEyq4zYAhfXnZ2dnUVZ_hJi4p2d@earthlink. com... I need to lose about 5 dB from a 15 Watt exciter. Thanks. Simple answer is no. While you can build one with wirewound resistors , the normal formulars will not usually work at RF and you will have a lot of inductance to deal with. Even the socalled non-inductive resistors are only so at the audio frequencies. Can the inductive reactance be cancelled with capacitors? I'm mainly concerned with 40, 30 and 20 meters. If your highest frequency is only 14MHz, and you're not trying to obtain a sooper-dooper perfect match for your exciter, then I'd say that you would certainly get away with using wirewound resistors. To minimise the effects of the inductance, you could try mounting the resistors 'hard down' against a ground plane. You might need a sheet of some insulating material to avoid any danger of shorting to it. And presumably, to get rid of 5dB (nearly 3/4 of your original 15W) you intend to use paralleled-up resistors. This in itself will help minimise the inductance. -- Ian A bunch of chip resistors in parallel to ultimately make a T-atten. Yebbut.... 12W is a fair bit to dissipate with chip resistors (even a lot of them). However, the question was if you could use WW resistors, not 'how to do it properly'. I presume there was a reason why this was asked! After all, we ARE talking 'amateur' radio. **** you. |
Can I use wirewound resistors to build an HF attenuator?
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Can I use wirewound resistors to build an HF attenuator?
In rec.radio.amateur.equipment dave wrote:
wrote: A 5db T attenuator with 15 W in has the maximum dissipation in the shunt resistor at about 5 W. The input series resistor dissipates about 4 W and the output series resistor dissipates a little over 1 W. I see no reason to go to wirewound resistors at these power levels. What kind of 5 Watt resistors should I use? Just about anything you can get other than wire wound. Mouser has metal oxide 5 W resistors for $0.49 quantity 1. You don't really want to run them at full rating but you will likely have to parallel two to get the values you need anyway unless you have a cheap source of better than 5 W precision resistors. You need two 14.01 Ohm resistors and one 82.24 Ohm resistor to build the attenuator. If you parallel two 27 Ohm for 13.5 Ohm, and two 150 ohm for 75 Ohm, you wind up with an attenuation of 4.95 db and a VSWR of 1.04 for a 50 Ohm system. -- Jim Pennino Remove .spam.sux to reply. |
Can I use wirewound resistors to build an HF attenuator?
A bunch of chip resistors in parallel to ultimately make a T-atten.
The whole point of wirewound is that I can make a T pad with 3 5 Watt resistors. They make "flange" attenuators but they cost a fortune in small quantities. Not really necessary for HF ops. The only advantage to using individual resistors, is to reduce the stray reactance. That goes way out the window with WW. Inexpensive solutions often revolve around buying surplus 1/2 watt or larger axial Carbons and arrange them in parallel to achieve the required values. This also helps to disperse the heat generated. The parallel resistors are soldered close to the body to Brass or Copper stock to improve dissipation. Also consider that although tune up will require 100% duty cycle, CW is 50% duty cycle and SSB is less. Check the Math in the Allied or ARRL book. |
Can I use wirewound resistors to build an HF attenuator?
In message , dave
writes Ian Jackson wrote: In message , JB writes "Ian Jackson" wrote in message ... In message , dave writes Ralph Mowery wrote: "dave" wrote in message news:0vqdnaEyq4zYAhfXnZ2dnUVZ_hJi4p2d@earthlink. com... I need to lose about 5 dB from a 15 Watt exciter. Thanks. Simple answer is no. While you can build one with wirewound resistors , the normal formulars will not usually work at RF and you will have a lot of inductance to deal with. Even the socalled non-inductive resistors are only so at the audio frequencies. Can the inductive reactance be cancelled with capacitors? I'm mainly concerned with 40, 30 and 20 meters. If your highest frequency is only 14MHz, and you're not trying to obtain a sooper-dooper perfect match for your exciter, then I'd say that you would certainly get away with using wirewound resistors. To minimise the effects of the inductance, you could try mounting the resistors 'hard down' against a ground plane. You might need a sheet of some insulating material to avoid any danger of shorting to it. And presumably, to get rid of 5dB (nearly 3/4 of your original 15W) you intend to use paralleled-up resistors. This in itself will help minimise the inductance. -- Ian A bunch of chip resistors in parallel to ultimately make a T-atten. Yebbut.... 12W is a fair bit to dissipate with chip resistors (even a lot of them). However, the question was if you could use WW resistors, not 'how to do it properly'. I presume there was a reason why this was asked! After all, we ARE talking 'amateur' radio. **** you. Dave, that's a very strange response. Do you have a problem with me suggesting that you probably had a reason for asking if WW resistors could be used? I've already said that you will probably 'get away with it' up to 14MHz, especially if you mount the resistors as close as possible to a ground plane. You may not realise that lot of 'engineering' is the art of 'getting away with it' (for whatever reason). For example, the reason might be economics, it might be practicality, it might be urgency, or it might be availability. The list is endless. I'm just putting myself in the situation where someone needs to lose 5dB of RF drive, and (say) all he has immediately at hand is a selection of WW resistors. Does he try them (knowing that they are not recommended for RF work)? Or does he take advice from the experts who, without a second thought, will probably say "No way!". The point is, do you want to try the WW resistors, knowing full well that they are not really the right thing to use, but will probably work well enough for what you want? Or do you want to do things 'correctly', knowing that what you have done will be almost perfect? You have received various bits of advice (some conflicting). It's now for you to decide what suits you best in the circumstances. -- Ian |
Can I use wirewound resistors to build an HF attenuator?
wrote:
In rec.radio.amateur.equipment dave wrote: wrote: A 5db T attenuator with 15 W in has the maximum dissipation in the shunt resistor at about 5 W. The input series resistor dissipates about 4 W and the output series resistor dissipates a little over 1 W. I see no reason to go to wirewound resistors at these power levels. What kind of 5 Watt resistors should I use? Just about anything you can get other than wire wound. Mouser has metal oxide 5 W resistors for $0.49 quantity 1. You don't really want to run them at full rating but you will likely have to parallel two to get the values you need anyway unless you have a cheap source of better than 5 W precision resistors. You need two 14.01 Ohm resistors and one 82.24 Ohm resistor to build the attenuator. If you parallel two 27 Ohm for 13.5 Ohm, and two 150 ohm for 75 Ohm, you wind up with an attenuation of 4.95 db and a VSWR of 1.04 for a 50 Ohm system. Thank-you. That's probably how I'll do it. |
Can I use wirewound resistors to build an HF attenuator?
Ian Jackson wrote:
In message , dave writes Ian Jackson wrote: In message , JB writes "Ian Jackson" wrote in message ... In message , dave writes Ralph Mowery wrote: "dave" wrote in message news:0vqdnaEyq4zYAhfXnZ2dnUVZ_hJi4p2d@earthlink. com... I need to lose about 5 dB from a 15 Watt exciter. Thanks. Simple answer is no. While you can build one with wirewound resistors , the normal formulars will not usually work at RF and you will have a lot of inductance to deal with. Even the socalled non-inductive resistors are only so at the audio frequencies. Can the inductive reactance be cancelled with capacitors? I'm mainly concerned with 40, 30 and 20 meters. If your highest frequency is only 14MHz, and you're not trying to obtain a sooper-dooper perfect match for your exciter, then I'd say that you would certainly get away with using wirewound resistors. To minimise the effects of the inductance, you could try mounting the resistors 'hard down' against a ground plane. You might need a sheet of some insulating material to avoid any danger of shorting to it. And presumably, to get rid of 5dB (nearly 3/4 of your original 15W) you intend to use paralleled-up resistors. This in itself will help minimise the inductance. -- Ian A bunch of chip resistors in parallel to ultimately make a T-atten. Yebbut.... 12W is a fair bit to dissipate with chip resistors (even a lot of them). However, the question was if you could use WW resistors, not 'how to do it properly'. I presume there was a reason why this was asked! After all, we ARE talking 'amateur' radio. **** you. Dave, that's a very strange response. Do you have a problem with me suggesting that you probably had a reason for asking if WW resistors could be used? I've already said that you will probably 'get away with it' up to 14MHz, especially if you mount the resistors as close as possible to a ground plane. You may not realise that lot of 'engineering' is the art of 'getting away with it' (for whatever reason). For example, the reason might be economics, it might be practicality, it might be urgency, or it might be availability. The list is endless. I'm just putting myself in the situation where someone needs to lose 5dB of RF drive, and (say) all he has immediately at hand is a selection of WW resistors. Does he try them (knowing that they are not recommended for RF work)? Or does he take advice from the experts who, without a second thought, will probably say "No way!". The point is, do you want to try the WW resistors, knowing full well that they are not really the right thing to use, but will probably work well enough for what you want? Or do you want to do things 'correctly', knowing that what you have done will be almost perfect? You have received various bits of advice (some conflicting). It's now for you to decide what suits you best in the circumstances. I know how to think. I have no resistors, period, except for the 1/4 Watt ones I use at work. Mr. Jim has pointed me in the right direction. All "amateur" means in the context of Amateur Radio is that we do not get paid for doing our thing. It doesn't imply that we are stupid or incompetent. I am fully cognizant of the possibility of meaningful reactance being introduced by a coil of Nichrome, and was hoping that someone who deals with passive components more regularly than I could point me in the right direction, and that has happened. |
Can I use wirewound resistors to build an HF attenuator?
dave wrote:
wrote: In rec.radio.amateur.equipment dave wrote: wrote: A 5db T attenuator with 15 W in has the maximum dissipation in the shunt resistor at about 5 W. The input series resistor dissipates about 4 W and the output series resistor dissipates a little over 1 W. I see no reason to go to wirewound resistors at these power levels. What kind of 5 Watt resistors should I use? Just about anything you can get other than wire wound. Mouser has metal oxide 5 W resistors for $0.49 quantity 1. You don't really want to run them at full rating but you will likely have to parallel two to get the values you need anyway unless you have a cheap source of better than 5 W precision resistors. You need two 14.01 Ohm resistors and one 82.24 Ohm resistor to build the attenuator. If you parallel two 27 Ohm for 13.5 Ohm, and two 150 ohm for 75 Ohm, you wind up with an attenuation of 4.95 db and a VSWR of 1.04 for a 50 Ohm system. Thank-you. That's probably how I'll do it. I got 10 of each, so I'll have some in the "miscellaneous" tackle box next time. Thanks. |
Can I use wirewound resistors to build an HF attenuator?
Mouser has metal oxide 5 W resistors for $0.49 quantity 1.
You don't really want to run them at full rating but you will likely have to parallel two to get the values you need anyway unless you have a cheap source of better than 5 W precision resistors. You need two 14.01 Ohm resistors and one 82.24 Ohm resistor to build the attenuator. If you parallel two 27 Ohm for 13.5 Ohm, and two 150 ohm for 75 Ohm, you wind up with an attenuation of 4.95 db and a VSWR of 1.04 for a 50 Ohm system. Thank-you. That's probably how I'll do it. Pretty good choice. I hate to buy from mail order suppliers because they often have a minimum order or minimum handling charge. I prefer to use what I can, especially for evaluation of a circuit. The formula for resistors in parallel is R = reciprocal of : sum of reciprocals of the selected resistors. So: R= reciprocal of 1/330+1/330+1/330+1/33= reciprocal of 4/330= 330/4 = 82.5 ohm for 4 ea 330 ohm 1 watt resistors for an 82.5 ohm 4 watt resistor Or for equal value resistors: The individual resistor values divided by the number of resistors 56/4 = 14 so a 14 ohm resistor can be made with 4 ea 56 ohm resistors. Now lets check the work: If the exciter is 20 watts and will see 50.04 ohms. The 50 of the amp input + 14 ohms is 64 ohms. If we parallel that with the 82.5 ohm... For 2 resistors in parallel R=( r1 x r2)/ (r1+ r2) = 36.04 ohm plus the 14 ohm in series =50.04 ohm to the input of the network. So the above values are correct for a T pad. Go to Ohms and Kirchoff's laws to see the current, voltage and power dissipation of the components: P=I squared R there is 632.5 ma through the 1st 14 ohm resistor for 5.6 watts, E=IR= 8.855 volts drop for the first 14 ohm resistor. The voltage for 20 watts at 50 ohms is SqRt of PR so 31.62 volts. The rest of the network after the 14 ohm resistor is 22.77v. The 82.5 watt resistor will see 6.283 watts of dissipation. Now current through the 14 ohm output resistor and 50 ohm input of the amp will be .3558amp By now you should have a copy of ohms law around. 1.77 watts is dissipated through the second 14 ohm resistor and 6.33 to the amp. You will notice all the dissipation of the amp and resistors adds up to 20 watts. The identical resistors in parallel will divide the power dissipation equally. Thanks for the exercise! I leave it to the rest to critique and cross check my math. |
Can I use wirewound resistors to build an HF attenuator?
In message , dave
writes I am fully cognizant of the possibility of meaningful reactance being introduced by a coil of Nichrome, and was hoping that someone who deals with passive components more regularly than I could point me in the right direction, and that has happened. From your other post, I see that you are going to use metal oxide resistors. In the circumstances, that's obviously the correct decision. But your question did not ask about what were the best resistors to use for the attenuator. It was "Can I use wirewound resistors to build an HF attenuator?" I assumed that you knew that WW would be inductive, but might have some reason for wanting to use them. Ralph Mowery replied that the "Simple answer is no" etc. You responded with "Can the inductive reactance be cancelled with capacitors? I'm mainly concerned with 40, 30 and 20 meters" which again made me think that you wanted to use the WW, if possible. I suggested that the effects of the inductance might be reduced if you mounted the resistors hard down against a ground plane. [This is a frequently-used technique for preserving the characteristic impedance, especially at higher frequencies.] JB then suggested "a bunch of chip resistors". I commented that you would need quite a lot of them. It was only then that I actually suggested that there might be a reason why you were asking about WW, and that the application was 'only' for amateur radio. Your response was "**** you". "How very strange", I thought! Anyway, I'm glad you have got yourself sorted out. I hope - no, I know - that the attenuator will definitely work as expected, with the advantage that it will probably work way up into the VHF region. -- Ian (licensed for 'amateur' radio 49 years ago this week) |
Can I use wirewound resistors to build an HF attenuator?
Ian Jackson wrote:
In message , dave writes I am fully cognizant of the possibility of meaningful reactance being introduced by a coil of Nichrome, and was hoping that someone who deals with passive components more regularly than I could point me in the right direction, and that has happened. From your other post, I see that you are going to use metal oxide resistors. In the circumstances, that's obviously the correct decision. But your question did not ask about what were the best resistors to use for the attenuator. It was "Can I use wirewound resistors to build an HF attenuator?" I assumed that you knew that WW would be inductive, but might have some reason for wanting to use them. Ralph Mowery replied that the "Simple answer is no" etc. You responded with "Can the inductive reactance be cancelled with capacitors? I'm mainly concerned with 40, 30 and 20 meters" which again made me think that you wanted to use the WW, if possible. I suggested that the effects of the inductance might be reduced if you mounted the resistors hard down against a ground plane. [This is a frequently-used technique for preserving the characteristic impedance, especially at higher frequencies.] JB then suggested "a bunch of chip resistors". I commented that you would need quite a lot of them. It was only then that I actually suggested that there might be a reason why you were asking about WW, and that the application was 'only' for amateur radio. Your response was "**** you". "How very strange", I thought! Anyway, I'm glad you have got yourself sorted out. I hope - no, I know - that the attenuator will definitely work as expected, with the advantage that it will probably work way up into the VHF region. That's way revisionist, but I am used to it. I apologize for the vulgarity. |
Can I use wirewound resistors to build an HF attenuator?
JB wrote:
Mouser has metal oxide 5 W resistors for $0.49 quantity 1. You don't really want to run them at full rating but you will likely have to parallel two to get the values you need anyway unless you have a cheap source of better than 5 W precision resistors. You need two 14.01 Ohm resistors and one 82.24 Ohm resistor to build the attenuator. If you parallel two 27 Ohm for 13.5 Ohm, and two 150 ohm for 75 Ohm, you wind up with an attenuation of 4.95 db and a VSWR of 1.04 for a 50 Ohm system. Thank-you. That's probably how I'll do it. Pretty good choice. I hate to buy from mail order suppliers because they often have a minimum order or minimum handling charge. I prefer to use what I can, especially for evaluation of a circuit. The formula for resistors in parallel is R = reciprocal of : sum of reciprocals of the selected resistors. So: R= reciprocal of 1/330+1/330+1/330+1/33= reciprocal of 4/330= 330/4 = 82.5 ohm for 4 ea 330 ohm 1 watt resistors for an 82.5 ohm 4 watt resistor Or for equal value resistors: The individual resistor values divided by the number of resistors Thanks for the exercise! I leave it to the rest to critique and cross check my math. If you go to RFcafe.com they have lots of on line calculators. I keep a scientific calculator on my desk, but the web is much faster. I have shortcuts to cable attenuation calculators, Ohm's Law calculators, LED resistor calculators, etc. I have a wall chart of dBm vs Volts at 50 Ohms. The above formula also works for capacitors in series. I knew that in my head. |
Can I use wirewound resistors to build an HF attenuator?
On Aug 18, 8:32*am, dave wrote:
I need to lose about 5 dB from a 15 Watt exciter. *Thanks. You could build the attenuator so it takes the power down in steps perhaps 2 1db attenuators followed by a 3db attenuator. I didnt run then numbers but I think this is doable with 2 watt resistors. Jimmie |
Can I use wirewound resistors to build an HF attenuator?
JIMMIE wrote:
On Aug 18, 8:32 am, dave wrote: I need to lose about 5 dB from a 15 Watt exciter. Thanks. You could build the attenuator so it takes the power down in steps perhaps 2 1db attenuators followed by a 3db attenuator. I didnt run then numbers but I think this is doable with 2 watt resistors. Jimmie Thanks. I have some honkin' metal oxide resistors that I'm going to use. |
Can I use wirewound resistors to build an HF attenuator?
wrote:
In rec.radio.amateur.equipment dave wrote: wrote: A 5db T attenuator with 15 W in has the maximum dissipation in the shunt resistor at about 5 W. The input series resistor dissipates about 4 W and the output series resistor dissipates a little over 1 W. I see no reason to go to wirewound resistors at these power levels. What kind of 5 Watt resistors should I use? Just about anything you can get other than wire wound. Mouser has metal oxide 5 W resistors for $0.49 quantity 1. You don't really want to run them at full rating but you will likely have to parallel two to get the values you need anyway unless you have a cheap source of better than 5 W precision resistors. You need two 14.01 Ohm resistors and one 82.24 Ohm resistor to build the attenuator. If you parallel two 27 Ohm for 13.5 Ohm, and two 150 ohm for 75 Ohm, you wind up with an attenuation of 4.95 db and a VSWR of 1.04 for a 50 Ohm system. Thanks, Jim. The thing works like a champ. I added a couple of $2.00 relays to bypass the pad when receiving. |
Can I use wirewound resistors to build an HF attenuator?
In rec.radio.amateur.equipment dave wrote:
wrote: In rec.radio.amateur.equipment dave wrote: wrote: A 5db T attenuator with 15 W in has the maximum dissipation in the shunt resistor at about 5 W. The input series resistor dissipates about 4 W and the output series resistor dissipates a little over 1 W. I see no reason to go to wirewound resistors at these power levels. What kind of 5 Watt resistors should I use? Just about anything you can get other than wire wound. Mouser has metal oxide 5 W resistors for $0.49 quantity 1. You don't really want to run them at full rating but you will likely have to parallel two to get the values you need anyway unless you have a cheap source of better than 5 W precision resistors. You need two 14.01 Ohm resistors and one 82.24 Ohm resistor to build the attenuator. If you parallel two 27 Ohm for 13.5 Ohm, and two 150 ohm for 75 Ohm, you wind up with an attenuation of 4.95 db and a VSWR of 1.04 for a 50 Ohm system. Thanks, Jim. The thing works like a champ. I added a couple of $2.00 relays to bypass the pad when receiving. Good to hear I didn't drop a decimal... -- Jim Pennino Remove .spam.sux to reply. |
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