EMRFD Message Archive 7524
Message Date From Subject 7524 2012-06-05 11:19:37 ha5rxz Minicircuits Bulk Buy I'm putting an order together for Minicircuits parts but some of them have a minimum order quantity, specifically:
GALI-84+ 21dBm output amplifier chip (16dB gain)
MOQ is 20 devices @ $2 each and I need 5
DAT-15R5-PP+ 0-15dB variable attenuator
MOQ is 10 devices @ $3.55 each and I need 3
Is anyone interested in taking the surplus parts off me once the order arrives? $60 for the lot including postage.
7525 2012-06-06 10:13:40 richj_focus Re: Minicircuits Bulk Buy Hello:
Those variable attenuators Are interesting. Do you know of a break out board for them to be able to use them in ugly construction?
7526 2012-06-06 19:34:01 Julio Jimenez Re: Minicircuits Bulk Buy Rich,
Not sure what packaging these attenuators come in, but check out
Schmartboards. They have different boards kits for a variety of SMD
packages. Ive used them to convert an FST3253 for use in a through-hole
board. Here's the link...
73 DE AK4VJ
7527 2012-06-06 22:00:03 Kerry Re: Minicircuits Bulk Buy I suspect that the pin layout (DG983-1 package) is a Mini-Circuits "exclusive". :(
One saving grace is that many of the pins are grounded; only a few are used for signal, voltage etc so some careful work with scalpel and heat may be possible. A detailed drawing is here;
Thanks Julio for the tip on Schmartboards; I hadn't heard of them. Even better, they are available here in VK
Another good & cheap source for these little boards is Futurlec;
but, alas, nothing to suit the MiniCircuits device.
7529 2012-06-16 18:08:46 Mike Self-resonant frequency of toroid inductors subject: prediction of self-resonant frequency of toroidal inductor
I have been looking for anything from a rule-of-thumb to a rigorous
analysis that would allow me to estimate the self-resonant frequency of
a toroidal (ferrite, or powdered iron) single layer wound inductor.
This is for use in a 3-30 mhz wideband circuit.
I don't know what the "parasitic" (residual) capacitance of the inductor
is in advance. And while I might be able to measure the self-resonant
frequency of the toroidal inductor after it is constructed, I'd like to
be able to predict the self-resonant frequency in advance of winding the
The application is for a unun (but not using transmission-line
construction; this is more like an autotransformer) that would provide
step-up in impedance from 50 ohms to several thousand ohms, for an
end-fed half-wave (~20 meter long) wire antenna, capable of handling
several hundred watts PEP from 7 to 30 mhz. I have found one possible
design for a similar application, at PD7MAA's web site. Considering the
power level that I want to handle, I expect that I will have to pay
special consideration to saturation of the toroidal magnetic material,
but my question does not deal with possible limitations imposed by
Does anyone have any suggestions on this? Ideally, I'm looking for a
rule-of-thumb (or equation) that predicts either the self-resonant
frequency, and/or the parasitic winding capacitance, as a function of
toroidal (a) dimensions (OD, ID, thickness, maybe core material too),
and/or (b) "other" parameters, i.e. frequency and/or wire size and/or
turn-to-turn spacing and/or number of turns and/or "winding gap" (i.e.,
the "gap" in degrees between the start and end of the winding; typically
30 degrees to 80 or 90 degrees.) Or any other parameters that are
particularly relevant (maybe spacing from the winding conductor to the
core; i.e. the thickness of the conductor insulation?)
Maybe the only available suggestions would be something somewhat crude
such as "figure 2 pf for OD < 0.5 inch, 5 pf for 0.5 < OD < 1.0 inch" or
something similar. Although I'm looking at cores in the 1.4 to 2.4 inch
OD size for my particular application.
I've tried searching the web for this info but so far no luck. Although
there are many sites that address how to determine (or measure)
self-resonant frequency if the parasitic (winding) capacitance is known,
or vice versa.
This might actually be a search for "fool's gold." Since a circuit
impedance of, say, 3000 ohms (which might be typical of the feeding
impedance for an end fed half wavelength antenna) at a frequency of 30
mhz equates to an equivalent of only single-digit picofarads, maybe it's
outrageous to expect to put together something of this nature. But
given the experience of the EMRFD group, I think somebody out there
might have some advice to offer.
7530 2012-06-16 22:59:15 David Stone Re: Self-resonant frequency of toroid inductors I think that you are trying to look for the wrong thing and trying to make an antenna matching device which won't work. I will explain:
The self resonant frequency of an inductor is simply controlled by the capacitance between the ends of the winding. In reality this may be made up of capacitance between each turn, but this is all summed up and a single self resonance frequency is generally found, best done with a network analyser. It s quite sensitive to the way that the winding is formed, the size of the wire etc, but as the core has a fairly high permeability is usually well away from the working frequency.
Now to antenna matching. Here we need to understand the maximum power transfer theorem, but not for the "impedance" you are discussing but for both the resistive and reactive parts that make up the impedance, which is a vector sum of these two components. Power is transferred efficiently when the resistive part is matched completely and the reactive part is zero, and this is the function of the ATU. Anything else will be seen as an unsatisfactory SWR, which really is measure of the impedance match. A large unmatched reactive component effectively prevents power transfer. So assuming you have a half wave resonant end fed wire, can you match it with a wideband transformer? The answer is in principle yes, but the feed voltage with several hundred watts will be very high and the transformer ratio must match the end resistive part accurately. Moving away from resonance, the reactive part becomes noticeable very quickly (because the antenna Q is high), the
required ratio to match the resistive part changes and the transferred power will rapidly drop to nearly zero.
So antenna tuners use resonance to allow matching of both parts of the impedance, even away from antenna resonance itself, to allow proper power transfer. They have adjustments (manual or automatic) because the exact values required change rapidly with frequency, and there is not a "wideband" solution.
If wide frequency range without adjustment is needed we can cheat, for example several dipoles of somewhat different length can be hung together to broaden response, or you can use a log-periodic beam. For best results these still need some matching adjustment. End fed wires are very difficult without an atu.
This is somewhat simplified, but probably sufficient see where to look further on the net
7531 2012-06-17 00:24:26 William Carver Re: Self-resonant frequency of toroid inductors A transformer doesn't fall into the category of providing a match for
any length of wire and anyfrequency. That requires a large degree of
adjustability in a matching network. But with due respect for what David
Stone presented, there CAN be a useful function performed by a
transformer in an end-fed antenna.
I have a top-loaded 53 foot vertical pipe which is series resonant at 3
MHz. It was designed to have its resistance pass through 50 ohms in the
80 meter band, where I use a series capacitor to resonate it.
It is also near halfwave resonance on 40m, slightly capacitive, and of
course the resistive component is several thousand ohms. Although it is
fed with Heliax, connecting this directly to a very long feedline to a
tuner in the shack had such high losses they were obvious even on
I was able to wind a toroidal coil on two T200-6 cores glued together
whose inductance was adjusted to make it resonant with the capacitance
of the antenna on 7 MHz. I was then able to tap up from the bottom end,
producing an autotransformer, to obtain a near-50 ohm impedance,
resulting in a SWR well under 2:1. Since the reactance of the coil was
near resonance with the reactance of the antenna, the reactance at the
tap is near zero.
So although the transformer cannot in itself provide a perfect match, it
made it practical to perform the final impedance match in the shack
without the high feedline loss. It produced quite acceptable results on
Although this was planned on paper first, I didn't try to predict the
stray capacitance of the coil ahead of time. I used a small toroid and
wire so the winding could be easily modified to obtain resonance. Then
its inductance was measured and I wound the final transformer with #12
AWG on the two toroids. Squeezing/spreading turns was used to obtain
zero reactance at 7.15 MHz and the computed tap point worked perfectly.
Regards - Bill - W7AAZ
7532 2012-06-17 05:26:15 David Stone Re: Self-resonant frequency of toroid inductors Yes of course what you say is correct, and the way that a transformer is correctly used. However my understanding of the question was to transform an unknown impedance to some convenient value say 50 ohms by simply using a fairly random ratio as high! Your use only attempts to match an unknown resistive part (measured by trial and error but this is a good way unless you are equipped to the teeth) as the reactance is tuned out (resonated) close to the working frequency. You probably have something else helping you too which is a fairly hefty ground loss resistance so the antenna Q is not very high. I remember a loaded vertical I once worked on at about 1.8MHz had a bandwidth of about 10kHz, and an incredibly low radiation resistance because it had a huge earth mat underneath and was matched with a transformer. Another point is that you wished to reduce feedline loss which is fair enough, but the post seemed to want just a wideband match without any sort
of ATU, and one would normally be used in the shack to match the rig for maximum power output, as many solid state PAs reduce the output with almost any SWR as a protective measure, even if the feed SWR were largely corrected by transformer.
7533 2012-06-17 06:36:05 William Carver Re: Self-resonant frequency of toroid inductors Your response, which I left attached below, is as spot-on as the
original one and I think we've covered the issue pretty well. The
transformer is suited for spot frequency use (depending on the antenna
Q) and only approximate match, especially since transformations are
limited to a ratio of integer numbers.
As it happens I have a rather extensive radial system and a pond
(although I don't think the fresh water does much good) but the vertical
is 6" in diameter so the Q is tolerable on 160m. On 80m, the original
design, and 40m where the transformer was used, one could cover the
whole band with only a modest rise in SWR at the band edges.
As for being equipped to the teeth, of course! My N2PK VNA, a netbook
computer, Dave's VNA software and a little gel cell battery are now
indispensible tools. It increases the value of EZNEC immensely!
> Yes of course what you say is correct, and the way that a transformer
> is correctly used. However my understanding of the question was to
> transform an unknown impedance to some convenient value say 50 ohms by
> simply using a fairly random ratio as high! Your use only attempts to
> match an unknown resistive part (measured by trial and error but this
> is a good way unless you are equipped to the teeth) as the reactance
> is tuned out (resonated) close to the working frequency. You probably
> have something else helping you too which is a fairly hefty ground
> loss resistance so the antenna Q is not very high. I remember a
> loaded vertical I once worked on at about 1.8MHz had a bandwidth of
> about 10kHz, and an incredibly low radiation resistance because it had
> a huge earth mat underneath and was matched with a transformer.
> Another point is that you wished to reduce feedline loss which is fair
> enough, but the post seemed to want just a wideband match without any
> of ATU, and one would normally be used in the shack to match the rig
> for maximum power output, as many solid state PAs reduce the output
> with almost any SWR as a protective measure, even if the feed SWR were
> largely corrected by transformer.
7535 2012-06-17 14:09:20 RW Mail List Re: Self-resonant frequency of toroid inductors Hi all,
You may like to also refer to Rudy N6LF excellent series of articles in QEX.
What interest me was figure 2 in part 3. Moving the four radials 6 inches up from the ground made a 5 dB increase in S21 transmission (to a receiver antenna), instead of heating up the ground.
Maybe for portable field use, the radials could be put on top of plastic tent pegs to hold them up in the air. Also different types of soil resistance at different portable locations wouldn’t change the input impedance. But you would need to put up a DANGER sign to stop people from tripping over the radials.
Also interesting was how a N2PK VNA was used to measure transmission S21.
I haven’t finished reading all of Rudy’s articles. But I would like to know of a method on how to separate and determine the Radiation resistance and Loss resistance parts of the input impedance. If it’s at all possible.
Roderick Wall, vk3yc.
7536 2012-06-17 15:25:19 kb1gmx Re: Self-resonant frequency of toroid inductors 7537 2012-06-17 15:44:56 kb1gmx Re: Self-resonant frequency of toroid inductors