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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?
|
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 |
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