13

What limits the use of "solid state variable capacitors" like these for dynamically filtering (low power/received) RF signals? The main issue is bandwidth. Although there are some varicaps that claim a 10:1 variation (or more) in capacitance, not all of this is useable. One end of the range may be low Q, or have a very steep curve. Once you have a ...


8

These were common abbreviations in use for Components, Radio equipment, and Printed publications in the 19th and most of the 20th century. Mfd = microfarad or μF kc = kilocycles or kHz. 1,000 cycles per second. μμf or mmf = pF (picofarad) Frequency The change from kc to kHz and mc to MHz were done because someone though that Heinrich Hertz ought to be ...


8

First, a general statement: the antenna analyzer has one set parameter, the frequency, and one measured parameter, the impedance (which is a complex number and therefore requires two real numbers to display). Everything else can be derived, one way or the other. Why for Z & Zpar are there 2 numbers, one with a j in front. What do those mean? $Z$ ...


6

Congratulations on your home brew project! It is great to see experimentation of this type. The two primary parameters for a capacitor are working voltage and ESR (effective series resistance), with the former being of primary interest in this case. How much power can be applied to a circuit containing a capacitor is a derivative of these parameters and of ...


6

As mentioned in the comment by henryflower, make sure that the shaft (and thus the frame) of the variable capacitor are grounded. You need to use rubber mounting grommets around the screws which bolt the tuning capacitor to the PCB or chassis. Then run one thick wire from the frame of the tuning capacitor from a point away from the front panel to one point ...


5

Technically, the utility of a variable capacitor in a receive filter depends on the amount of available capacitance variation and its Q - the quality factor, which is the ratio of the capacitor's reactance at the operating frequency to the series parasitic resistance. (In some applications, parasitic series inductance may also be a factor.) Commercially, ...


5

Welcome to StackExchange. Your questions are natural for a new user of an instrument like the AA-35. Some of your questions are addressed in the item on Impedance in Wikipedia. Impedance, denoted as $Z$, describes two aspects of a circuit's behavior when stimulated with AC: resistance, $R$, which dissipates energy, and reactance, $X$, which stores and ...


5

Feed-through capacitors are still commercially available but their use in commercial applications has fallen off so prices have risen over the last few years. The concept with the feed-through capacitor was to provide an RF capacitive bypass to ground while providing a stout connection through the wall of the enclosure. When I see them at a swapfest for a ...


4

The most important and simplest thing you can do to eliminate alternator whine is run a separate set of power cables directly from the cars battery terminals to the radio. Making sure you include the proper rated fuses links on both leads immediately after the battery terminal connection. (the fuses are for safety and should be on both the positive and ...


4

Circuit looks OK. Note that the highest impedance point in this circuit is near the box open end. RF voltage here is highest in this oscillator, and is susceptible to coupling out the top. A shield lid (soldered to the box) should help.


4

Another difference between using a varicap for oscillators versus for input filters is feedback. You can feed the output of a variable oscillator into a phase comparator or frequency counter to determine if the component values are correct, and run a feedback loop correction algorithm or circuit to maintain lock. This can be done while the oscillator is ...


3

Alternator whine is caused by a small amount of AC signal component on your DC power supply to the radio. The alternator generates this AC signal as a by-product of how it works. A diode pack on the alternator converts AC current from it's windings into DC to charge the battery. The battery "smooths" out the pulses of energy from the alternator. For more ...


3

The reason to use a bypass capacitor instead of a simple capacitor to ground is that you'd typically want to keep the series inductance to ground as low as possible. So take this model of a capacitor with parasitic series inductors: simulate this circuit – Schematic created using CircuitLab and make it something more like this: simulate this circuit ...


3

From the ARRL Handbook... Spacing inches ___ V_peak 0.015 ___ 1000 0.02 ____ 1200 0.03 ____ 1500 0.05 ____ 2000 0.07 ____ 3000 0.08 ____ 3500 0.125 ___ 4500 0.175 ___ 7000 0.25 ____ 9000 0.35 ___ 11000 0.5 ____ 13000 Also note that these are mere recommendations. Actual high voltage you will see of course depends on the reactance and input power to ...


3

Great idea. Why not just make a single two-wire transmission line, called an open circuit stub, and cut it to length with a knife. $Z=-jZ_0Cot(\beta l)$ or $C=1/{\omega Z_0 Cot(\beta l)}$ where $\beta=2 \pi/\lambda$ and $\omega = 2 \pi f$ and $Z_0 ≈ 100 \Omega$ for a pair of 1 mm lines 1 mm apart, but use an online calculator to find a more accurate ...


2

As others have pointed out, the discrete components, suchs as capacitors and inductors, start to be quite challenging in higher frequencies. In general, transmission lines tend to have lower losses and higher repeatability around your frequency. Long story: While the transmission lines might be better for practical applications, I wanted to share the ...


2

Sure, if your L/C meter is designed to work at those capacitances. The capacitance of a gamma rod isn't much -- it's probably measured in pF. The lowest range of the inexpensive L/C meter I have from eBay is 200pF, which is fine for most of the capacitors in my parts drawer, but not so great for RF. Also keep in mind: if you are making connections with long ...


2

To expand on Glenn's answer: The ESR, i.e. the losses in your capacitor that aren't caused by ohmic heating of the copper but by conversion of change of electrical field to heat in the dielectric (i.e. the FR4 and air) can be written as $$\text{ESR} = \frac{\tan \delta_\text{FR4}}{\omega C}$$ with $\tan \delta\approx 0.01$ being the material loss ...


2

Maximum amperage of the capacitor may also be limited by the smallest feed wire in the path and by the thickness and width of the metal in the plates of the capacitor, and possibly the skin depth of the components as well. (In other words, if the skin depth for the frequency you are using is too close to the thickness of the metal in your plates, the ...


2

Perhaps you are thinking of a switched capacitor filter? Unfortunately, most switched capacitor IC components are (were) designed for audio frequencies, not VHF. Another possibility is to use an LC tuned circuit containing a voltage controlled varicap diode capacitor, controlled by a (digital input) DAC. As per the earlier answer, you can use a use a ...


1

Consider increasing the dynamic range of the ADC to obtain many of the same performance benefits. This can be done by either upgrading the ADC, or low-pass filtering the ADC input below the Nyquist frequency and oversampling. Often this is the cheaper and more flexible approach, especially given the limitations of the varicap's Q-factor and capacitance range,...


1

A "simulated inductor" might work in your application: You could adjust the simulated inductance by using an analog multiplexer IC and resistive ladder array IC to change the values of RL and R1.


1

Would an inductor with SRF > 7 GHz and Q > 10 be okay at above frequency? Could it be made to work? I suppose. But any discrete inductor at 6 GHz is going to have significant non-ideal behaviors that you will need to model (or empirically measure) to get an accurate match. At the SRF the inductive reactance and capacitive reactance are equal, but this does ...


1

They are almost certainly capacitors, probably a few pF each. If it were a bigger name, you could probably find a schematic, but that seems improbable here. A high SWR could be caused by a shorted capacitor, put a continuity meter across them. If it reads short, one of them is dead. If it reads open, they're probably OK. If the caps seem OK, the antenna ...


1

You raise an interesting point; intuitively, what I'd have thought is that the peak voltage matters – because discharge through the dielectric doesn't happen for any reason but an electrical field strength (at least for vacuum as dielectric, which can have no electric polarization of any kind, being not a material) above the Schwinger limit which you will ...


1

The R7000 and 7000+ are NOT quarter wave verticals they are HALF wave verticals( check dimensions you'll see starting 10 meters that its not 8ft=quarter wave , its about 12-13 ft , shortened from 16ft because of the traps)., the spokes at the base are NOT radials they are Counterpoises, 3 feet distance is not enough above metal roof or even an AC unit(I ...


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