WHEN DO YOU NEED TO USE A MATCHBOX?
[ Are you sure you even need a matchbox? ]
WHEN SHOULD YOU USE A MATCHBOX ?
ANSWER: Hopefully Never! . . . because ALL matchboxes add unwanted losses!
But in reality: In general, you need to use a matchbox whenever the impedance of the antenna and its feedline (the antenna system) does not match the impedance of the transmitter (usually 50 Ohms). For instance:
- On 80m where the band is simply too wide to be covered by a single antenna.
- When you wish to feed the antenna with openwire feedline.
- When using multi-band antennas (generally a compromise).
- e.g., G5RV, Windom (also called Off-Center-Fed Dipole or "OCF"), multi-band antennas without adjustments for the individual bands, etc.
- When you purchased a transceiver with a poorly designed finals stage, where the power folds back with higher SWR.* (will be explained below)
- When you purchased a Linear Amplifier which trips to fault when the SWR exceeds a pre-determined level (which is often too low to be practical).** (will be explained below)
WHEN IS IT BETTER NOT TO USE A MATCHBOX ?
1. As often as possible.
- ALL antenna matchboxes induce additional loss to the antenna system. With good matchboxes it is less than 10%, but not all matchboxes are good - and NO MATCHBOX is good on every band with every impedance. NEVER BELIEVE THEIR MARKETING. Use 3rd party resources, such as ARRL Lab tests to determine a matchbox's effeciency. (See MATCHBOX SHOOT-OUT for more information) ...........................................
2. Whenever you are using monoband antennas.
- With monoband antennas, it is usually possible to fix the antenna instead of using a matchbox. This is always the most efficient solution - because ALL matchboxes add additional loss to the system.
RULES TO FOLLOW:
- NEVER USE A MATCHBOX IF YOU DO NOT ABSOLUTELY HAVE TO !
- Switch it to "BYPASS" or "THROUGH" when it is not absolutely needed.
- IF YOU HAVE TO USE A MATCHBOX, BE SURE YOU USE ONE THAT IS WELL SUITED FOR THE JOB. Remember, different matchboxes have different Sweet-Spots.
- Otherwise you may incur excessive power loss within the matchbox.
( THE CONTENTS BELOW ARE MY PERSONAL OPINION. )
* Many Transceivers, especially those built in Japan, fold back their output power in the presence of high SWR to prevent damage to the final transtors.
The question here is, "what is the definition of high SWR?"
- Some transceivers fold back the output power when the SWR exceeds 2.5:1.
- This is reasonable but still not good, especially if the SWR problem is just an instantaneous one, due to the antenna swaying in a breeze.
- GOOD TRANSCEIVERS will operate all day long into an SWR of 3.0:1 without folding back their output power. Personally, I prefer to use a good transceiver, not one where the manufacturer saved a few dollars on the final amplifier stage.
- Other transceivers fold back their output power by as little as 2:1 SWR.
- This is totally unacceptable and a clear indication of an under-dimensioned final amplifier stage.
- Many people simply accept this as "normal" because the don't realize that there are transceivers which have much better designed final amplifier stages.
- Transceivers like these are totally unacceptable to me, but that's my opinion.
There are good transceivers on the market which will deliver full power, all day long, into an SWR of 3:1. (For instance, any TEN-TEC transceiver). Many other "better transceivers" can also deal with 3:1 SWR. Most of the lower cost transceivers cannot. That's why they are cheaper.
MOST PEOPLE DON'T EVEN REALIZE THERE ARE DIFFERENCES HERE!
Any time you have to use a matchbox in your antenna system, you are inserting additional losses. It is always better not to run a matchbox if you don't absolutely have to.
** Many Linear Amplifiers are equipped with fault monitoring circuitry which among other things, trips to fault whenever the SWR exceeds a value which is dangerous for the amplifier. Though in principle a good idea, this is not necessary when running a well designed amplifier within legal power limits on the amateur bands. In fact, the protection circuit itself can also cause unnecessary trouble.
Unfortunately most modern amplifiers fall into this category. That's why I personally prefer amplifiers using "vintage" glass tubes, such as the 3-500z. The 3-500z tube is very rugged and most amplifiers using this tube do not have fault (do not need) trip circuitry.
Avoiding amplifiers with fault trip circuitry enables you
to avoid the unnecessary need to use a matchbox.
The problem with SWR fault monitoring is, especially in stormy weather with antennas blowing in the wind, you can have instantaneous (non-dangerous) swings in the SWR which will trip the high SWR fault circuitry of many modern amplifiers. In reality these are harmless swings in the SWR because they immediately swing back to normal, and would not damage the amplifier. Never-the-less, the amp goes into fault mode and switches itself into standby position.
For this reason, I prefer to use amplifiers without SWR fault monitoring/warning.
The reason we have SWR fault monitoring in many modern amplifiers is, many modern tubes can blow almost instantaneously under certain over-current conditions. It is simply too dangerous (and too expensive) to run them without SWR fault warning.
For this reason, I refuse to buy and use one of these modern amplifiers.
MOST PEOPLE DO NOT SHARE MY OPINION.
(They won't buy me a new tube, either.)
In my 50+ years on the air, I have owned 8 linear amplifiers which used the 3-500z tube(s). Knock on wood, (touch wood if you're British) I have never blown the tube(s) in any of these amplifiers. That's why I stick with amplifiers that use the 3-500z.
*** Most Transistor Linear Amplifiers use fault monitoring / fault warning circuitry which trigger instantly in the presence of high SWR, but there are a few transistor amplifiers that do not. Instead these other amplifiers monitor for excessive heat (which can also be caused by longer periods of high SWR). So they also fault with high SWR, but only if it is for an extended period of time. They don't fault instantly with short swings in the SWR. In my opinion this is good enough. I have had good luck using this type of amp in contests. Unfortunately most transistor linear amps don't work like this. They fault much too soon!
IN MY OPINION, most transistorized linear amplifiers are good for normal ham radio use, great for portable use, but a very poor choice for serious contesting.