I’m planning to make a mono-band LC-matched antenna for 30m. The calculators that I used suggest a 5.5uH shunt inductor and 47 pf series capacitor will give me a <1:1.5 SWR in the 10.100 to 10.130 range.
Given that I plan to use a QMX or QCX for this, I wish to minimise losses. Will there be a significant difference in losses between using an air core inductor vs a fixed inductor? Or am I over thinking this and should just use a 49:1 unun and be done with it?
Annas
No significant difference. But have tune your LC. On the other hand you have to cut precious your wire for resonance. More tolerable by using a LC tuner.
73 Chris
Most folks use a tuned transformer if they want to go resonant matcher, Annas. M0UKD seems to be the guru-in-residence: End Fed Half Wave Antenna Coupler (EFHW) – M0UKD – Amateur Radio Blog
If I understand him correctly, with a decent cheese slicer in there, you can cover almost 3:1 frequency range - not an important criteria for you, I know, but if it comes free…
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That’s difficult to say, because a fixed inductor might be either air core, or use a powdered iron, ferrite, or brass slug.
Basically, there shouldn’t be a significant difference between using an air core or a powdered iron core (probably a toroid using red #2 or yellow #6 material, or equivalents). The powdered iron cores reduce the size, but don’t get too carried away and use too small of a wire size.
Either way you do it, it’s good to be able to adjust the position of the turns on the coil to get the proper value.
With an L network, both components must have the proper values to obtain a good match. With the parallel tuned circuit (still just one coil and one capacitor, although the coil needs either a tap on one of the turns, or a link winding) either the coil or capacitor can vary over a fairly broad range, as long as the other component is adjusted accordingly. I have generally used a small trimmer capacitor for this purpose, after making sure that it can handle the voltage at the power level I plan to run.
When I converted a base-loaded CB (27 MHz) mag mount to a 2m half wave, the variable capacitor made it easy to retune the antenna between 2m ham and 155 MHz for Search and Rescue. But if you don’t need that feature, then either approach should work.
As Chris said, efficiency-wise there shouldn’t be a difference that’s worth considering. But you may want to be able to adjust the match for changing environments, antenna heights, etc. A simple and effective matchbox is by Steve Yates AA5TB coupler for EFHW. I am using this and find it useful to be able to adjust, compared to my fixed coupler. More useful for the QMX perhaps that seems more sensitive to poor SWR.
73 Peter
Don’t worry about losses as long as high-voltage capacitors (blue caps for 1, 3, 5 kV or more) are not used, the efficiency of an LC match is typically around 95..97%.
The use of fixed L-matches is unproblematic as long as the short leg of such extremely off-center fed antennas hangs freely in the air, i.e. does not touch the ground with its random influences and is not attached anywhere (e.g. ~0.5 lambda of the outside of the shielding braid of a coaxial feeder cable).
The trimmer capacitor in series connection enables a stress-free initial adjustment and is not touched again afterwards.
By the way, this implementation was posted earlier in an EFHW thread, along with a similar one by Ignacio, EA2BD …
Can you elaborate on that? I think you mentioned it elsewhere, but I would be interested in more details. Thanks.
