L match atu range - Page 1
Forum section: electronics / rf-microwave

author: p.larner
author_rank: Frequent Contributor
timestamp: December 30, 2023, 11:33:25 am
content: i am building an l match atu with a 1-1 balun for windowline fed antennas,the roller coaster is 50uh max and the cap 1000pf max wide spaced,what is the impeadance range i can expect it to match or swr missmatches,it will be switched for high or low Z.

author: A.Z.
author_rank: Super Contributor
timestamp: December 31, 2023, 10:38:41 pm
content: do you have a VNA ?!?

author: vk4ffab
author_rank: Frequent Contributor
timestamp: January 01, 2024, 08:08:52 am
content: Quote from: p.larner on December 30, 2023, 11:33:25 amAt what frequency?An L match is essentially a reactive low pass filter, series inductor with shunt capacitor. What it will match will depend on the minimum inductance and capacitance values as well as its maximum that you have given. Given that the minimum capacitance will dominate what can be matched at higher frequencies at a guess without knowing what the minimum values are and without simulating the circuit, it might match 50R into 10K ohms at 3Mhz dropping to 500 ohms at 28Mhz. (ASSUMPTION made minimum capacitance is about 25pf) Realistically, you want a much smaller value capacitor, like 300 puff max with as little minimum capacitance as possible for better response on the upper bands, but if all you care about is 80 40 and 20, then what you have should be fine ISH.Download RFsim99 and play with values for a L match, or better yet, learn how to use LTspice or other circuit simulator.

author: A.Z.
author_rank: Super Contributor
timestamp: January 01, 2024, 09:11:26 am
content: vk4ffaba way to allow covering a widest range of R/X is to use a rotary switch to insert fixed capacitors in series or in parallel with the variable, so allowing to reach quite low/high capacitance values, by the way the whole matching network should also be well built, keeping the wirings short to minimize stray capacitance/inductance

author: Hamelec
author_rank: Regular Contributor
timestamp: January 01, 2024, 09:15:48 pm
content: All attempts to explain are pointless...

author: p.larner
author_rank: Frequent Contributor
timestamp: January 05, 2024, 12:22:14 am
content: well this is now built,it works well from 80 to 20m,any frequency above 14megs it wont match,i imagine there is too much min capacitance,i was going to remove plates from the cap,but thinking about it could i put another variable cap in series with the 1000pf cap already fitted instead?,would that help match the higher hf bands?.

author: vk4ffab
author_rank: Frequent Contributor
timestamp: January 05, 2024, 03:18:54 am
content: Quote from: p.larner on January 05, 2024, 12:22:14 amWhat is the minimum inductance of the roller and the minimum capacitance of the cap? Measure both values, as well as sweep the antenna from say 15 to 29Mhz and post what the impedance's are on 15, 12 and 10m. From there, armed with all the information, we can work up a plan. Without actual data, its hard to say what to do or what the cause is or if it is practical to worry about it. You have enough space in your loft for a shorter antenna and that might be the better option after all things are considered.All i can offer is what I would do, more than that, I just don't know.

author: A.Z.
author_rank: Super Contributor
timestamp: January 05, 2024, 04:51:10 pm
content: Quote from: p.larner on January 05, 2024, 12:22:14 amWhy a variable ? It will suffice to add some switchable, fixed capacitors, but for sure sweeping the antenna with a VNA to get the R+X curves on the various bands will allow to calculate the needed min and max values for the L and Cthe schematic below, assuming that the 365pF variable has a min capacity of 10pF, allows to obtain a capacity range between 3.75pF and 1065pF, the fixed capacitors should be able to support at least 1KV, if needed, it is possible to add another switch to insert fixed inductors in series with the roller, as designed, if my math wasn't wrong the capacity ranges should be the following3.75 5.90 C5 series5.23 10.67 C4 series10.00 365.00 C1 (VC)360.00 715.00 C2 parallel710.00 1065.00 C3 parallelso allowing to cover the whole range w/o "holes", the rotary switch should be a two section one with ceramic insulation, the rotary and the other switches should be able to withstand the voltage generated by the transmitter (or amplifier)oh and, btw, the wiring should be kept short, to minimiza stray inductance/capacitance, but that's something ANY SERIOUS builder knows

author: vk4ffab
author_rank: Frequent Contributor
timestamp: January 05, 2024, 09:01:37 pm
content: Quote from: A.Z. on January 05, 2024, 04:51:10 pmIt still might not extend the range if the roller inductor has a minimum of 1.5uH. The assumption I made early in this thread was the inductor went into the low nH range, it might not, many common 50 mike rollers only go as low as 1uH ish. That will still be useless on the high bands even if you drop the minimum capacitance to 5 puff.We just need more information. Like what sort of load it needs to match on each band and how low both the cap and inductor will go.

author: MathWizard
author_rank: Super Contributor
timestamp: January 06, 2024, 04:21:07 am
content: I'm trying to learn calculating impedance matching. And a whole lot of times they go off into S-parameter's, and probably way higher frequency considerations that I'm doing for AM radio ATM. I'm getting more familiar with coax cables and the physics, but for a few MHz or dozens of MHz, and under 1meter of RG-58, IDK the right way to use it yet or calculate what I want.If you don't have a VNA or whatever, and you calculate or measure ABCD parameter's, then convert them to S parameter's, is that acceptable, or is there more to it ?Also most of the examples I've tried have been to maximize power transfer at 1 frequency. But if you're doing something "simple" like impedance matching an amplifier to the 50 input of a scope, well IDK all the combo's yet to do want I want when the input capacitance of the BJT is already fixed.

author: p.larner
author_rank: Frequent Contributor
timestamp: January 06, 2024, 09:41:40 pm
content: ive a nanovna but am pretty clueless on using it otherthan sweeping swr,at the moment i have removed half of the caps vanes and stator,if that is no good on 14megs up shall i switch in a small coil instead of the roller coaster,on pulling the cap apart one of the ceramic supports for the fixed plates was busted,just glued it with epoxy so waiting on it to cure before i reasemble and test,73.

author: vk4ffab
author_rank: Frequent Contributor
timestamp: January 07, 2024, 01:52:39 am
content: Quote from: p.larner on January 06, 2024, 09:41:40 pmThis will make your life enjoyable:https://nanovna.com/?page_id=90it makes measuring Z easy mode.