17
votes
Accepted
How did hams manage to tune their antennas before VSWR meters?
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 ...
- 7,864
11
votes
Accepted
Why aren't "ghosts" and intersymbol interference due to unmatched impedance (high SWR) a concern for HF receivers?
Intersymbol Interference (ISI) — or analog equivalents, like ghosting — are relevant only when the difference in time of arrival between the primary and reflected signal are significant compared to ...
- 51.6k
10
votes
Accepted
RF signal level when the antenna is disconnected
The situations described, schematically:
simulate this circuit – Schematic created using CircuitLab
When the source and load are matched (Rantenna = Rreceiver), you will be measuring the ...
- 51.6k
10
votes
Impedance ratio vs. SWR
As you can see from the following equation, it is definitely not that easy. What I would do is draw a 10:1 SWR circle on a Smith Chart and assume that your tuner can match all of the infinite number ...
- 1,646
9
votes
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Do I need a separate SWR meter if the transceiver has one built-in?
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 ...
- 51.6k
9
votes
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Does a halfwave (or multiple) length of coax duplicate the load impedance of one end at the other end?
I have been told that 50 ohm coax is 50 ohm at both ends, and 50 ohms all along its length, and it is 50 ohms that is presented to anything to which it is connected, and the length of it does not ...
- 11.8k
8
votes
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Exactly why do some SWR meters give a changing reading depending on the length of coax used to connect to an antenna?
The SWR is related to the reflection coefficient $\Gamma$:
$$ \Gamma = {Z_L - Z_0 \over Z_L + Z_0 } $$
$$ \text{VSWR} = {1+|\Gamma| \over 1 - |\Gamma|} $$
where:
$Z_0$ is the feedline impedance, ...
- 51.6k
8
votes
RigExpert AA-35 - Interpreting The Information
First, a general statement: the antenna analyzer has one set parameter, the frequency, and one measured parameter, the impedance (which is a complex number and therefore requires two real numbers to ...
- 24.4k
8
votes
What is antenna reactance?
Reactance is the imaginary part of impedance. Together with the real part, resistance, impedance describes how a given load will respond to an AC voltage or current source. While for DC analysis ...
- 51.6k
8
votes
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Do you need to analyse the SWR of your radio repeatedly? Or periodically? If so, Why?
If nothing about the physical situation changes, then the SWR will not change. But something might change in the long run, perhaps:
Damage to the feed line (water intrusion, sun damage to insulation, ...
- 24.4k
7
votes
Coax cable as antenna for 5 watt handy
The electrically shortened antennas often found on HTs are not simple pieces of wire, but coils:
By Shootthedevgru at English Wikipedia, CC BY-SA 3.0
The coil adds inductance over the length of the ...
- 51.6k
7
votes
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How do I set up my radio for an antenna tuner?
Your TS430S has a solid state final and no internal antenna tuner. As such it is designed to work into a 50 ohm load at all times. This means you can disregard this note in your antenna tuner manual.
...
- 18.5k
7
votes
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Trying to understand the work of 4:1 coax balun
Think of it first without the feedline:
The half-wavelength of transmission line provides a 180 degree phase shift, and the (short) connection between the shield ends is "ground". That means it's at ...
- 51.6k
7
votes
Accepted
Can I use a directional coupler to block reflected AC current from a mismatched antenna?
There is a device that does this, but it's not a directional coupler. Rather, it's a circulator.
A directional coupler isn't suitable since the forward and reflected ports merely "sample" the power ...
- 51.6k
7
votes
Are "end-fed" antennas really more multi-band than dipoles?
End fed antennas do work on more bands than centre fed dipoles.
The reason is that end fed antennas are resonant on all harmonics, while normal dipoles only resonate on every other harmonic.
To see ...
- 359
7
votes
Are "end-fed" antennas really more multi-band than dipoles?
It's mostly real.
To a first approximation, a center-fed dipole will have a reasonable impedance on bands where it is an odd multiple of a half wavelength (the fundamental, third harmonic, fifth ...
- 11.8k
6
votes
Accepted
Does Coax Limit the Maximum SWR a Transmitter 'Sees'?
The source of the SWR limit on the transmitter end is the losses in the feedline. In general, the higher the matched line loss, the lower the maximum SWR that will be present at the transmitter end. ...
- 18.5k
6
votes
Impedance matching: Why do components behave totally differently from theory?
Firstly, at 900+ MHz, shorting out components to take antenna impedance measurements is not a good practice as the lead length can be a substantial portion of a wavelength. Stray inductance and ...
- 18.5k
6
votes
Impedance Matching between RF Amplifier Stages
There is no need to "pin" an interstage impedance. You may directly transform from the native output impedance of one stage to the native input impedance of the next without going through an ...
- 18.5k
6
votes
Using 75 Ω instead of 50 Ω coax feed
Sure, you can use 75 ohm coax instead of 50 ohm. In some cases (such as feeding a dipole), 75 ohm coax may be a better match to the load than 50 ohm. In other cases (such as feeding a vertical), it ...
- 51.6k
6
votes
Accepted
Better SWR bandwidth
A low SWR bandwidth is a consequence of shortening the antenna for mobile use. SWR bandwidth is related to the antenna system's Q factor, the ratio of energy stored to energy dissipated per cycle. A ...
- 51.6k
6
votes
How to match low impedance of NVIS Dipole?
Many baluns will work just fine in either direction, though there isn't just one kind of "4:1 balun".
This kind is wound on two cores, and works as a common-mode choke:
simulate this circuit – ...
- 51.6k
6
votes
Accepted
If receive performance doesn't depend on SWR, then how can I tune my manual HF antenna tuner by band noise?
Receive power does vary with SWR; but for reception quality we're interested in signal-to-noise ratio. As long as you're getting enough receive power that the noise received from the antenna is ...
- 11.8k
6
votes
Accepted
Estimating SWR based on multiples of resonant length
Very high. Harmonics work the other way around — antennas can be useful on higher bands than the design band, but almost never lower bands. A dipole cut for 40m might be good on 15m. An end-fed or a ...
- 11.8k
6
votes
Accepted
Is testing SWR *practically* less important for 2m/70cm than HF bands?
I would say in practice it is less important because hams use different types of antennas for HF and for VUHF.
At HF, the same antenna is often used for many bands. The bands have large fractional ...
- 10.5k
6
votes
Accepted
Does an SWR higher than 1:1 necessarily cause additional heat in the transmitter?
A mismatched load might make the transmitter hotter, but it might also make it relatively cooler. It might also precipitate other failure modes, such as flashes, smoke, bangs, oscillation, spurious ...
- 51.6k
6
votes
Accepted
Impedance miss-match short vs long section?
Hams usually send just one frequency down a transmission line (as opposed to say high speed data like HDMI). So it's much simpler to analyse impedance down the line, than look at the reflections. ...
- 10.5k
6
votes
Reasons for an antenna tuner at the transmitter other than transmitter protection?
Just because the finals don't go up in smoke when driving a mismatched load, doesn't mean they're working well. They will probably deliver less power to the antenna than they would in a matched ...
- 11.8k
6
votes
cut or lengthen to reduce swr on vertical wire antenna
If you're only measuring SWR at one frequency, or in a narrow range like the ham bands, you won't know until you cut it.
Mount the antenna in its final location as the SWR and resonant frequency will ...
- 10.5k
6
votes
Is there a proper way to connect multiple radios via coaxial cable so they can be tested without going over the air?
My company built a system like this, to allow real radios to be used in a flight simulator. It had fixed and digital variable attenuators, so it could simulate exactly the effect on communication of (...
- 10.5k
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