MATCHBOX POWER LOSS
[ ALL Matchboxes have Power Loss ]
RULE OF THUMB:
ALWAYS ASSUME THAT WHEN YOU USE A MATCHBOX
YOU ARE AUTOMATICALLY GIVING UP
AT LEAST 10% OF YOUR POWER.
YOUR 100w is now only 90w!
(if you are lucky)
Don't be surprised if YOUR matchbox has even more loss!
THIS IS WHY YOU SHOULD NEVER USE A MATCHBOX
UNLESS YOU ABSOLUTELY HAVE TO.
MATCHBOX LOSS FACTS:
- ALL MATCHBOXES HAVE LOSS. SOME HAVE VERY HIGH LOSS.
- Power loss can be less than 10% but it can also be 50% or more!
- THE AMOUNT OF LOSS VARIES BY BAND AND FREQUENCY.
- Generally losses are highest on 160m and 10m
- This is because it is not possible to optimize a matchbox for both ends of the hf spectrum.
- THE AMOUNT OF LOSS VARIES, DEPENDING ON THE IMPEDANCE OF THE ANTENNA SYSTEM.
- Generally losses are highest in all matchboxes at low impedances (less than 20 Ohms).
- But losses may also be high for high impedances in some matchboxes.
- SOME TYPES OF MATCHBOXES ARE LESS EFFECIENT THAN OTHER MATCHBOXES.
- The popular T-Matchbox usually has higher loss than other types, and . . .
- . . . quite often is mis-tuned resulting in even higher losses.
- Yet it is still popular due to its broad impedance matching range. - BUT JUST DON'T USE IT UNLESS YOU ABSOLUTELY HAVE TO!
- USUALLY, EFFICIENT MATCHBOXES HAVE A LIMITED IMPEDANCE MATCHING RANGE.
- By limiting the matching range, we can use components optimized for one specific range.
- GENERAL PURPOSE MATCHBOXES HAVE A BROAD IMPEDANCE MATCHING RANGE, BUT ALSO HAVE MORE LOSS THAN OTHER TYPES OF MATCH BOXES.
- This is the trade-off we often accept in order to have our "All-in-Wonder Matchbox", capable of matching everything from the bedsprings to the Eifel Tower in Paris.
- IT SEEMS YOU CAN'T HAVE YOUR CAKE AND EAT IT TOO.
- If you want to have a good matchbox for all bands and all impedances, you will need to buy or build 2 or 3 matchboxes.
The ARRL has measured Antenna Tuner (matchbox) losses for many common matchboxes. If you want to know how much loss your matchbox has, perhaps it has been tested by them. See MATCHBOX SHOOT-OUT for this.
WHY MATCHBOXES HAVE LOSSES:
FIRST OF ALL, EVEYTHING THAT RF PASSES THROUGH HAS LOSSES, NO EXCEPTIONS!
THE LOSSES INSIDE OF THE MATCHBOX ARE DUE TO:
- THE SIZE AND QUALITY OF THE COMPONENTS USED
- For the lower bands, especially 160m, we need a lot of capacitance and especially a lot of inductance. In order to obtain a high Q (to reduce loss), we would need a very large coil. Since a matchbox the size of a washing machine will not fit on our desktop, we use smaller components and accept a compromise. (slightly exagerated, but you see the point)
- For the 10m band, we need very small values of capacitance and inductance - typically smaller than the minimum capacitance of the variable capacitor. If we make the variable capacitor smaller, we will have worse efficiency on the low bands. It's a trade-off.
- THE COMPROMISE WE ACCEPTED FOR THE PURPOSE OF ACHIEVING A BROAD IMPEDANCE MATCHING RANGE, RESULTS IN ADDITIONAL LOSSES.
- We can improve the effeciency (reduce the loss) by focusing on a smaller matching range and then selecting components optimized for that range. The efficiency goes up, but the usefulness of the matchbox goes down. As I said, it's a trade-off.
I will stop there and give you a link where you can read more.
DOWNLOAD: GETTING THE MOST OUT OF YOUR T-NETWORK ANTENNA TUNER.
The following file is also available on the ARRL Web Site as a public download. You do not have to be an ARRL member to download this file.
This document was written by Andrew S. Griffith, W4ULD, and published in the January 1995 issue of QST.
Getting the most out of your T-Matchbox.[...]
PDF-Dokument [406.9 KB]