# Tag Info

18

Dispelling the Myth To begin with, the typical HF SWR meter does not have the ability to separately sample the forward and reverse power, voltage, or current. Any description of the device or its circuitry that suggests this capability is flawed. We can show this empirically with two different experiments. Experiment 1 Connect a 100 ohm resistor directly ...

16

Mostly, they used RF Ammeters in series with the antenna. The first ones were hot-wire ammeters which were completely mechanical devices. One end of a thin nichrome wire (or other wire of sufficiently high resistance) inside the meter was coupled directly to the pointer shaft (often wrapped around it); the other end was anchored to the meter case. As the ...

10

At power levels that low, you probably don't need an SWR meter permanently installed. You will find it useful to have an SWR meter or antenna analyzer available when building the antenna, though, to ensure it is properly operating on your chosen frequencies. Perhaps you can borrow one from a friend, mentor, or local radio club to use while building your kit?

10

A cross-meter is capable of showing you three measurements simultaneously: Output Power Reflected Power SWR From this image by Axel Schwenke on Wikipedia, you can see that the needle on the left indicates forward power, and the needle on the right indicates reflected power. The observed intersection of the two needles can be used to indicate the SWR of the ...

8

You'll be fine to start without an additional SWR meter. An SWR meter doesn't provide any protection. With or without an SWR meter, you'd want to start on a new antenna on low power, then increase power only after measuring the SWR. Don't worry too much. If transmitting at much less than maximum power you won't damage anything even with the worst possible ...

7

I've not personally built antennas from scratch, but I appreciate my antenna analyzer just for being a good instrument — making the invisible aspects of my antenna system visible. Compared to using a SWR meter for the purpose, an antenna analyzer: Displays more information. A SWR meter still gives you enough information, in the sense that you can try ...

6

There's no problem with what you're proposing, usually. Most of your other equipment probably has exposed metal connected to “ground”. If it's not causing trouble, this won't either. But if you have the problem known as “RF in the shack” — your transmitter's RF coming back to you on the outer shield of your feedline — then the metal will be RF-hot, but you ...

6

A properly calibrated cross-needle power meter such as e.g. the MFJ-842 actually tells you something more than just the forward and reflected power, which as you point out can just as easily be indicated by two separate instruments. The intersection of the needles gives you a pretty good indication of the actual standing wave ratio or SWR because the SWR is ...

5

Given the matched loss of the feedline and the SWR at the transmitter, we can calculate the SWR at the antenna in three simple steps. First convert the SWR at the transmitter to the corresponding magnitude of the reflection coefficient (Gamma), or MRC for short within the context of this answer. The MRC is the magnitude of the complex ratio of the reflected ...

5

Many hams have re-purposed CB SWR meters but there are so many variations that some caution and testing is warranted. Many CB meters were not designed to handle the higher power levels of amateur transceivers. A 100 watt transmitter could cause component failures within the meter. While the most likely result is simply the failure of the meter, I wouldn't ...

4

This should not be a problem at all. It is commonly done this way. Hopefully the switching supply has been designed for ham radio use. If not, you may find some "birdies" on your receiver which are remnants of the switching frequency of the supply getting into your receiver and potentially interfering with desired signals or simply causing annoying tones (...

4

The calibration procedure is only used for determining the SWR - the forward and reflected power reading are not impacted. If you change power levels or frequency, you need to perform the calibration again to read the SWR. The calibrate procedure is to set the needle to the CAL position on the meter, not to the 100 watt reading as you described. Then flip ...

4

For an ideal circulator, all the power entering one port exits the next port in the rotation and no other. So all the power in port 1 exits port 2, all into port 2 exits port 3, and all into port 3 exits port 1. This is expressed by the scattering matrix: $$S={\begin{pmatrix}0&0&1\\1&0&0\\0&1&0\end{pmatrix}}$$ Or a 4 port ...

4

Mainstream hams first started to appreciate SWR in the 1940's. In that era some of the first SWR meters started to come on the market. But their accuracy and frequency range was quite limited. Toward the end of the 1940's military surplus coaxial cable was hitting the ham market. Up to that point, ladder line was typically used for most ham antenna ...

4

I don't know much about this, but for what it's worth I do have a rather distant memory of doing this long ago, before transistor RF power amps became affordable and before I had a way to measure SWR or harmonic content of the transmitter output. The transmitter was a very basic VFO, buffer and a 6V6 RF final, maybe 3 or 4W RF output max. The antenna was ...

4

VSWR is related to the magnitude of the reflection coefficient $\Gamma$: $$\text{VSWR} = {1+|\Gamma| \over 1-|\Gamma|}$$ The reflection coefficient can be calculated from the load impedance $Z_L$ and the characteristic impedance $Z_0$: $$\Gamma = {Z_L - Z_0 \over Z_L + Z_0}$$ The reflection coefficient, like the load impedance, a complex number with ...

3

An antenna analyzer is not necessary. Sure, it's handy. A VNA is even handier. But then, these things could set you back maybe \$100 if you go for the cheapest antenna analyzer, or maybe thousands of dollars if you go for a modern VNA. If you just want to know if you need to trim a bit off a wire dipole or not, and simply get on the air without toasting your ...

3

There are many ways you could calibrate. For measuring SWR, try this: Attach a 50Ω dummy load to your transmitter. The SWR should be 1:1. Then, attach two 50Ω dummy loads in parallel, giving you an effectively 25Ω load. The SWR should be 2. It seems you also want to measure power, and not just SWR. If you have some attenuators available, you can put them ...

3

A balun has two functions: present a high impedance to common-mode currents present a low impedance to differential-mode currents A balun that does just the first but not the second is easy: an open circuit fits the bill. Testing with an SWR meter doesn't test the first, but it does test the second. That does mean the balun is doing at least half its job. ...

3

A 1:1 choke-balun has a design impedance, usually 50 ohms. It also has an electrical length, e.g. mine is 60 degrees long on 28 MHz (4 feet of RG58 on two FT240-52 cores). If I load it with a 50 ohm dummy load on 28 MHz, it will read ~50 ohms on the input because the SWR is 1:1. However, if I test it with a 600 ohm dummy load, it will read 5.8-j26 ohms on ...

3

The repeating hi-Z measurements seen when SWR is measured with the 30m line suggest that it is defective. One needs to know the velocity factor and loss of the cable to determine the location of the defect. These should be determined from measurement, as actual lines can deviate significantly from published specifications and values can change as a result of ...

2

I've been a ham since 1963 and have built all my own antennas. Yagi beams—wire beams—loops—verts, etc. Back in those days all you had was a SWR bridge and maybe a grid dip meter. The most important thing, I think, is a booklet or some kind of articles on antenna design. They will give you accurate lengths and feed line info that should get you up and ...

2

I couldn't live without my two antenna analyzers -- I may even get a VNA (vector network analyzer) although that is not necessary for simple antenna analysis. I own both the MFJ 259B and the Autek Research RX Vector Analyst VA1. Although both do roughly the same thing, they have a few different features and I will touch on a few of those. Besides digital ...

2

To add to Glenn's excellent advice, some "CB" SWR meters will handle the legal limit on HF with ease. For example, I have two or three old Radio Shack meters that will do that. I have compared them with other meters of high quality (such as the wattmeter in my Collins 312B-4) and they are plenty accurate enough from 1.8 to 30 MHz. Having said that, I ...

2

Incomplete answer — I still haven't figured this one out, but here's some leads/ideas I have so far. I found a circuit with some similarities on Wikipedia called a single transformer hybrid coil: It is described as follows: X, Y, and Z share a common ground. As shown at left, signal into W, the 2-wire port, will appear at X and Z. But since Y is bridged ...

2

Well, I'm a little out of my depth, but this does seem to have all the usual parts of a gamma match: the matching section hanging off the main loop is a balanced transmission line, and the part connected to the BNC center is spaced off it, so the coax is coupled capacitively into that transmission line. There are two transmission lines, and two gamma ...

2

In a way, an SWR measures voltage and current simultaneously, and that's how it's able to separate forward and reverse power. W2AEW has an excellent video on how directional couplers work. One way to convert an SWR meter into an ammeter is to disconnect one of the transformers. Older equipment included an RF ammeter because that older equipment used vacuum ...

2

With the exception of 10 meters, the frequencies of lowest return loss (and SWR) measured with the VNA are below the ham bands on which you must have tested with the IC-7300. It's not easy to see on the graph, but the return loss within the ham bands appears to be about -5dB, which corresponds to an SWR of over 3:1. It's important to compare apples to apples ...

2

You should trim it so that the SWR is the same at each edge of your band segment of interest. If you tune it for the lowest SWR in the middle, you likely will find that the the above condition does not occur and the SWR might be too high at one edge.

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