How can I safely transmit without an antenna tuner or SWR meter?

Question

Just what the title says.

An Antenna Tuner, henceforth AT, is almost a de facto piece of equipment in a shack; working the bands without one is tantamount to leaping off the diving board into a swimming pool with tarantula nests in it ... or worse.

The reason for the AT is primarily to match the antenna to the transmitter for the Standing Wave Ratio. The greater the SWR, the more the risk to the transmitter. But before one uses an AT, one either designs the antenna (balanced/unbalanced, array-type, length, impedance ... and constructs it), or purchases the antenna.

Keeping it simple

• Say, A simple mono-band centre-fed dipole is constructed after calculating the length for that band
• Say further, neither an AT nor an SWR meter is available

Given the above assumptions, what I would like to know

• How can I transmit without an antenna tuner?
• Are there any rule-of-thumb tests/calculations I may do to determine whether an antenna is a decent fit for a given band?

As a corollary

• What if the antenna is not a simple centre-fed mono-band antenna?
• How did they tune antennas back when the hobby was still new? I guess Does a tube based HF transmitter need an antenna tuner? may be relevant to this part of the question

Answer 1

The SWR meter helps you match the impedance of the radio to the antenna. If the impedance is mismatched, you lose power. If the impedance mismatch is large, you risk damaging your radio, particularly on the lower frequencies.

Tube based transmitters and amplifiers have more leeway for mismatch than semiconductor based amplifiers.

Lower power transmitters also have more leeway for mismatch before damage occurs.

The ideal is to borrow an SWR meter and tune your antenna for the intended frequency, or send the antenna to someone who can do that for you.

If you don't have any of these tools and can't get help from others easily, you can get as close to the right frequency by building the antenna according to the design, then start transmitting on low power and make contacts. Find someone willing to work with you, and ask for signal quality reports. Then make a small adjustment to the antenna and ask for another report.

It's a long process, but they will receive more signal the better your radio is matched to your antenna, so it's an easy check.

Do this at low power though, so you reduce the risk of damaging your transmitter.

Answer 2

How can I transmit without an antenna tuner?

Simple. You use an antenna that's already tuned. There are plenty of radios that operate without any tuner. For example, basically every VHF radio. One reason for this is that most VHF antennas are purchased rather than manufactured by the amateur, and the antenna manufacturer has already tested and tuned the antenna design.

It's also relevant to mention that an antenna tuner doesn't actually make the antenna tuned. With a perfectly matched antenna (SWR 1:1), all the power sent down the feedline by the transmitter will be accepted by the antenna and radiated away. When the antenna isn't perfectly matched, some of the power is accepted by the antenna, and some is reflected back at the transmitter. When it reaches the transmitter, the transmitter's RF amplifier sees an impedance other than the 50 ohms for which it designed, which can mean currents or voltages high enough to cause damage. Some radios sense this condition and reduce output power to prevent damage.

By inserting a tuner between the transmitter and the feedline, the power reflected back from the feedpoint is then reflected again back at the antenna. The transmitter now sees no reflected power: it sees a well-matched load. However, the power is still being reflected back-and-forth between the feedpoint and the tuner, encountering losses in the feedline each time. So, the tuner doesn't make the antenna work any better: it just takes some load off your transmitter.

Answer 3

The key piece of information required to know if an Antenna Tuner is required is the SWR. If the SWR is higher than 1.5, you probably need an antenna tuner. If not, they it's not really a requirement. The SWR changes based on frequency, so you need to keep that in mind as well. SWR can be measured by various meters, just keep your eyes opened for one.

Okay, so what can cause the SWR to change? A lot of things really. The antenna needs to be at least a wavelength off of the ground to have optimal performance. It should be far away from metal objects. It might even change based off of local topology, and mineral content. These considerations are especially true for HF, which is why HF often has an antenna tuner. However, for VHF/UHF, the use of a tuner is rare, I certainly don't use one.

Answer 4

Reduce Power

One of the bad effects of SWR is heating of the power amplifier inside the radio. This heating occurs because of reflected power. The higher the SWR, the less power is transmitted out the antenna and more of the power is reflected back to the radio to become heat.

You can reduce reflected power by reducing output power. For instance, operate a 100W radio into an unknown antenna with the power set to only 5W.

Answer 5

Question by OP: did older tube based transmitters need an antenna tuner?

Back before the mid-1970s the antenna tuner was a more rare part of the ham radio equipment. This is because the Pi-network output of the tube based transmitter acted as an antenna tuner as you did your load/tune work to tune up the PA to match the load (the antenna). Thus, in a way, it was an antenna tuner but it was never called that.

The first antenna tuner I saw was in the early 1960s and it was the balanced line tuner by E.F. Johnson called the Matchbox Tuner. This antenna tuner was designed for multi-band wire antennas (such as doublets, loops) fed by open-wire ladder line (often 600 ohm ladder line).

During my early Novice days as a ham and my rebel days prior (starting in the 6th grade) as a bootleg CW operator (circa 1958-1959), I never had an antenna tuner, never had an SWR meter (didn't even know what SWR meant), and I operated quite a few DX stations on 40 watts of power and a crude non-resonant (on anything) dipole up in the air.

Yes, tubes were definitely more robust than transistors with respect to dealing with high-SWR but that was not the main reason that tube-based transmitters worked without (separate) tuners -- basically the Pi-network was their tuner.

Answer 6

A few comments regarding elements of your question(s) that I haven't seen addressed.

Think of the antenna wire and the feedline leading from your radio to the antenna feedpoint as all being part of your antenna system. Each component contributes to the overall efficiency of the antenna system. If the antenna system is designed to be efficient (low-resistance wires and low-loss feedline), and the impedance it presents at the radio's antenna connector closely matches the impedance the radio is designed to feed (typically 50 ohms resistive), then there isn't any need for an antenna tuner. In fact, an antenna tuner under those conditions would only add resistive losses to the overall antenna system making it less efficient.

Regarding "Does a tube based HF transmitter need an antenna tuner?" the answer is sometimes. Obviously if the antenna system is like the one described in the previous paragraph, no, no antenna tuner should be used. But when the impedance presented to the transmitter by the antenna system differs sufficiently from 50 ohms, a tuner might be required in order to avoid high currents (and associated heating) in the final output stage of the transmitter. Tube-type transmitters tend to be quite robust, in terms of the amount of heat their finals can dissipate without being destroyed, and thus can tolerate greater mismatches at full output power. Modern solid-state rigs automatically throttle back their power in order to prevent their more delicate transistors from being destroyed.

Multi-band antennas can be designed to present reasonable antenna system impedances across many bands. No antenna tuner is required for such antennas when they are used within their designed bands.

A well-designed quality antenna system is an excellent substitute for an antenna tuner. And a good substitute for an SWR meter when evaluating antenna matching across a band of frequencies is an antenna analyzer, such as those popularized by MFJ and others. In fact such analyzers are often much more convenient for that purpose, and provide more useful data than a simple SWR meter.

Answer 7

OP Question: Given the above assumptions, what I would like to know

• How can I transmit without an antenna tuner?

There is no law requiring an antenna tuner. Most solid-state rigs can handle a mismatch up to about 3:1 SWR. Above 3:1 there is usually severe fold-back of output power (most modern solid state rigs) to protect the PA from damage. Fold-back usually begins around 2:1 though and I know it does in my own Elecraft K3. However, by the time the mismatch is 3:1, so much power is folded-back that you are probably operating a QRP (even under 5 watts) station so you should get QRP credit (assuming you were transmitting 100 watts to begin with).

But, you can have a perfect dipole and still have high-SWR problems. Case in point. My 80-meter dipole (part of 80/40 fan dipole) has a 1.7:1 SWR at 3.57 MHz. I designed this antenna to operate best on the low-end of 80. My penalty is that the SWR gets worse as I go up in frequency such that by the time I am tuned to 3.9 MHz, the SWR is about 4.5:1 which is not that good (though easily corrected by an antenna tuner).

Therefore, my low SWR antenna at 3.57 is no where close to low SWR at 3.9 MHz.

On the other hand, my 30-meter dipole (operates 10.1 to 10.15 MHz band) is almost flat across the band. SWR at 10.105 MHz is 1.28:1, at 10.125 MHz it is 1.33:1, and at 10.145 MHz it is 1.38:1. So, I can (and do) use my 30-meter dipole without any antenna tuner switched into the transmission line circuit.

So, by careful design (and testing after it is in the air) of a dipole antenna and trimming or extending wire legs as needed for your desired frequency, you can have an antenna that has no need for an antenna tuner. Alternative methods exist in using transmission line transformers but this is often more problematic (in my opinion) and it does not really change the bandwidth of lower SWR you have to operate safely.

My examples are drawn here from my own experience with wire antennas because those are the most popular and they are my favorite. I do a lot of portable operations, especially when on birding photography trips with my wife (the photographer) I will set up a portable station out in the middle of nowhere using (usually) wire antennas I launch into trees with a sling shot. My SWR is NEVER good in these situations. I have no intent or attempt to create a low SWR resonant antenna. Instead, I make sure my transmission line is reasonable efficient and short (under 50 feet) to limit the losses. My rig most of the time is an Elecraft KX3 though I also use a KX1 and the Norcal 40A. Mode is always CW. Power is always QRP.

Answer 8

1) How can I transmit without an antenna tuner? -As mentioned by several answers, you can transmit without a tuner if the SWR isn't too high (I do it all the time.) I'd also add that even a resonant antenna can still have an SWR because the impedance doesn't match the transmitter.

2) Are there any rule-of-thumb tests/calculations I may do to determine whether an antenna is a decent fit for a given band? -For a simple mono band center-fed dipole, the rough formula is

l=433/f
where:
l is in feet and;
f is in MHz.

If the antenna is not simple then I don't even bother with calculations and go with trial-and-error.

3) How did they tune antennas back when the hobby was still new? -Other pieces of test equipment no one has mentioned are a noise bridge and a grid dip meter. (IIRC, an antenna analyzer is essentially a combination of the two with some other stuff thrown in, but I may be wrong.) You can tune antennas with those as well.

Answer 9

•How did they tune antennas back when the hobby was still new?

This part of your question has not yet been answered. The technique used was to set up a transmission line consisting of parallel wires to an antenna. It is possible to use a hand-held meter along the transmission line to determine the relative voltages on the line at different points of the line. The line exhibits standing waves on the line, that is, the peaks and lows are steady in place and can be found and measured. The ratio of the high voltage to the low voltage is the standing wave ratio - S.W.R. Adjusting the antenna also affected the SWR, as we know. When the peaks and valleys of the voltage differences evened out, and there were no more peaks or lows, then the SWR was 1:1 and the antenna was "tuned".

Answer 10

•How did they tune antennas back when the hobby was still new?

Answer moved to a new question here.

Answer 11

I do not think that most modern Antenna Tuners are all that great at reducing TVI, as most of them are NOT PI- Networks. Most of the newer tuners are T- Networks, which tune just fine, but are inherantly Hi Pass filters!

So if your transmitter doesn't contain some good low pass filters, the radio will put out a lot of energy outside the intended frequency range

Answer 12

You Can transmit on an unknown system ???? not very well! nor with any Real power . Simple answer to this is ! buy an SWR meter Or TUNER with ONE built in. The tuner can be switched out if the VSWR is below 1.3 and you use 100 W or less. TUBE AMPS AND radios are more forgiving but still require a tuner to Get the signal into the wire to be heard. . Signals NEED a resonant Antenna system to be heard you cannot simply RUN FORM 3.5 to 3.995 without A tuner the antenna length Varies by 8 meters in the length of each of its 2 Elements. same on 40 meters the Length required at one end is too much for a single antenna to comprehend under a 2.5 to 1 VSWR A:though you will learn how to Achieve a Better limit than 2:1 20 M is about the Longest Wavelength you can compromise your antenna on CB and one half of the 10 Meter Band can be covered without a tuner. VHF UHF are Ok BUT you ALWAYS need a SWR meter Capable of Accurate reading at HF VHF UHF no one meter will do it all. Ham Radio IS an Expensive Hobby if you don't have money to spare for these Items Don't start. Cut your antennas For the longest frequency of the band its for. NOT mid band and PLEASE USE A TUNER its a Great HARMONIC filter ( TVI filter) VE7IBV (Advanced Class 25 year)