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Cushcraft R7 RFC Value
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by KD4RBG on March 11, 2008
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Hi all,
Would any of you happen to know the value of the RFC coil in the black box on the Cushcraft R7?
I have been to every R7 site referenced by Google and eHam, but unless I've overlooked it, nobody is giving the value of that coil. It is referenced merely as "RFC" by http://www.iol.ie/~bravo/r7_vertical.htm. That site suggests just disconnecting it, but I'd rather replace things and keep them working as designed.
All the other parts check out as to their capacitance/inductance and the toroids aren't damaged.
Thanks,
Jeff/KD4RBG
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RE: Cushcraft R7 RFC Value
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by N3OX on March 11, 2008
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Do you have a reason to suspect it is bad?
"RFC" basically means "this has more than 10 times, maybe 100 times the impedance of whatever we're connecting it across"
So if you're measuring it with an RF impedance meter like an MFJ-259B, it should look open. Z> 650 ;-)
If you really do need to replace it (DC open) then what you want to do is figure out what is the minimum inductance you'd need to get something like 100 times the impedance of what it's connected across.
The "RF Choke" radio shack used to sell was something like 100uH, which is 1100 ohms at 1.8MHz. You have to watch out for the self resonance of big chokes, and that kind of sets the upper limit on them. The actual value is pretty non-critical though.
They should be reasonably easy to measure at audio frequencies or at least at 100kHz or so... but hard to measure at RF because of what they're designed to do.
73,
Dan
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RE: Cushcraft R7 RFC Value
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by K0BG on March 12, 2008
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The R7 is a loaded 1/2 wave antenna, and rather than a ground plane, it has decoupling stubs. In the box are more than just a single choke. There are two transmission line transformers; the two ferrite cores. One is an UNUN, the other a choke. There's also a small choke and cap. If there was a nearby lightning strike, the small choke might be bad, but that doesn't happen often. If you use a tuner to extend the bandwidth, you can also blow this little choke. They warn you about that in the manual.
The major problem with the antenna is mounting it too close to other objects. If you do, you'll never get it to work correctly. And when I say close, less than 10 feet or so, and even then you might have tuning problems. A restricted space antenna, it is not!
Alan, KØBG
www.k0bg.com
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RE: Cushcraft R7 RFC Value
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by N0NOB on March 12, 2008
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Jeff:
Look here, value for RF choke give BUT it is for a R5.
Not sure if they are similar.
http://www.mrs.bt.co.uk/mrs/r5/
Good Luck!
Irv
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Cushcraft R7 RFC Value
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by N0IU on March 12, 2008
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This site will tell you EVERYTHING you want to know about the R5/R7 series:
http://www.iol.ie/~bravo/r7_vertical.htm
73,
Scott N0IU
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RE: Cushcraft R7 RFC Value
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by N6AJR on March 12, 2008
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we rebuilt a friends R7 a while back, itt needed 2 new capicitors in the magic box. they sent them easy enough. the other problem was the plastic ( delrin?) insulators inside the matching stubs are no longer available, and you can try to repair them, but we hand no luck ( lots of effort, no results.)
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RE: Cushcraft R7 RFC Value
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by KD4RBG on March 12, 2008
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I love hams. Only on a ham board would somebody tell you to check a website that you specifically said you already checked.
For those of you who posted useful answers and more questions, I'll try to respond.
Irv, that was the page that prompted me to dig further. They call out 470uH, I believe, and my part reads 68uH. It was the only page with a value for the RFC choke and I thought I'd come here and ask for comparisons, because I was sure I wasn't the first to have this problem. Would you personally choose to replace it, or since it's just DC grounding the whole box, would you just take it out and forget it?
Alan, it's an Ebay special, so God knows what has been done operationally with it. Like I originally said, I disassembled the PCB inside the matching network box and I've tested the parts individually on an Agilent 4263B LCR Meter. Everything read within tolerance, *except* possibly the RFC, for which I have no authoritative reference for value and could only find one site that took a stab at it.
Which brings me to Dan...the reasons I suspect it may be bad are above. If a "standard" RFC value (which is really my question) is around 100uH like you posted, then maybe my RFC isn't the problem since it's reading about 68uH. If it's supposed to be 470uH, then some of the turns are obviously shorted.
I suspect this RFC also because the website that I initially posted calls it out as causing "High SWR across all the bands due to shorting". What's actually happening is the resonant point of my antenna is shifted down in frequency, and that is what I'm trying to resolve. I wish things were more precise, "High SWR" could mean several different things.
Back to Alan...didn't you tell me some years ago that in general, an entire antenna system should be at DC Ground? This is the part that causes that condition in this particular antenna...yet I can't see what downside there would be in just leaving this out if I can't find out for certain what value it's supposed to be.
Folks, I'm an engineer, and I've got an entire lab of stuff with which to play, none of which says "MFJ" on it. If you've got some way to calculate what this should be by taking measurements on the entire system, I'm up for trying it.
Dan, when you say "you'd need to get something like 100 times the impedance of what it's connected across", at what frequency do you mean? Would you take impedance measurements at all band centers, choose the highest impedance, multiply by 100, and then do the whole impedance-over-omega thing to find the value of the coil? And since Xl and Xc cancel, does that mean it's safe to go with this?
Looking forward to your replies,
Jeff/KD4RBG
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RE: Cushcraft R7 RFC Value
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by KD4RBG on March 12, 2008
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Tom,
I saw where several folks had replaced the caps with doorknobs of the same value. If mine go out, I may try that. Mine are holding, so far.
I don't suspect the traps yet because the resonant frequencies of each band are lower than can be adjusted out. If it was just one band, I might start looking there.
There was something related to fixing the traps with a hot glue gun, but I hope I don't have to do that. In fact, if it was a band I almost never use (like 30m) and it didn't throw off the rest of the system, I'd probably just let it ride...I don't know if Laird will sell you new traps but I think someone said they wouldn't. I might prefer new ones for this if mine go out
Thanks for the response,
Jeff/KD4RBG
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RE: Cushcraft R7 RFC Value
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by N3OX on March 12, 2008
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""you'd need to get something like 100 times the impedance of what it's connected across", at what frequency do you mean? "
Well, any frequency you're using, but at the same time, 100x is an awfully conservative "open circuit" ;-) If it's 68uH right now and it's about that at 1.8MHz, it's something like +j770 ohms. If it's connected across the 50 ohm side of things, 50 ohms in parallel with +j770 is 49.9+j4.0 or so, hardly giving you a bad SWR... so that's a factor of 15 or so in impedance magnitude and it doesn't cause much trouble.
If it's connected across the *200 ohm* side you get 187+j48... still only a 1.2:1 SWR or so with respect to 200 ohms. However, you DO need to know what the frequency dependence of the impedance is. I'd sweep across HF and measure if you can, because that's what's really important. The 68uH you measure at low frequency might be close to what it is on low bands and would be plenty of impedance, but it could be the original coil had self-resonances in certain spots that weren't harmful to the operation of the antenna, but if you have shorted turns, maybe they moved?
"Which brings me to Dan...the reasons I suspect it may be bad are above. If a "standard" RFC value (which is really my question) is around 100uH like you posted, then maybe my RFC isn't the problem since it's reading about 68uH. If it's supposed to be 470uH, then some of the turns are obviously shorted.
"
Yeah, if you're using Agilent gear to measure it and it's supposed to have a low freq inductance of 470uH I'd say it's got a problem but no one seems to know that... I bet Cushcraft does.
- - - - - -
Anyway, I'd try the antenna without the choke and see what happens. The original quip about 100x the thing it's connected across shouldn't be taken very literally because it's really awfully hard to do that if you're connecting across 200 ohms ;-) You just need enough impedance that the voltage across and the current through the choke doesn't destroy it or sap power from the antenna.
10x or 15x is pretty easy, should generally be sufficient for this particular antenna, and a choke wound on ferrite could easily hit 2000 or 3000 ohms over all of HF.
But you need to know the coil's impedance over the whole HF spectrum... you need to know if it stays inductive over the whole thing... whether or not any resonances that lower the impedance occur in the ham bands, and so forth.
The low frequency inductance of a little coil with hundreds of turns probably doesn't mean much at all at 28MHz.
73
Dan
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RE: Cushcraft R7 RFC Value
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by KD4RBG on March 12, 2008
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Dan, you're always right there with the right answer...I think rather than posting here I'm just going to email you directly in the future when I have a question, since it's going to be you that has the answer.
Seriously, though, I do appreciate it. I've gleaned a good deal of information from reading your archived answers to others, too.
It's a crying shame that in an engineering-based hobby there are so few engineers or even real technicians...I bet there aren't five people who regularly post here who really know what you mean by +j and -j, let alone being able to manipulate complex numbers.
I've got the box back together and I'm going to try it out without the DC grounding (with RFC removed). I will report back with any notable results later in the week.
Jeff/KD4RBG
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