22 votes
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How does an SWR meter really work?

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 ...
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13 votes

I have 60 feet of coax but only need 20 feet; can I loop up the excess?

There's nothing inherently wrong with “looping up” extra coaxial cable. In fact, a neatly wound coil of coax can function as an air-core choke balun (“ugly balun”) which is useful for some antenna ...
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13 votes
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Is there an optimum Antenna Feedline Length for mobile installations?

In the absence of common-mode currents, then the optimum feedline length is 0, because a longer feedline only increases your feedline losses. These losses are due to the resistance of the wire, ...
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10 votes
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Why is my calculation for added length of coax for a double cross antenna different to everyone else's?

The difference is the "Velocity factor". A 36cm long physical coax wire of this type is electrical 54.4cm long. Different types of wire have different velocity factors. https://en.wikipedia.org/wiki/...
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  • 126
9 votes
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Is "low loss" cable required for HF?

Your question seems as much about psychology as much as technical concerns. We mainly favor the technical questions, but I'll take a stab at the psychological aspects also. All of you, please feel ...
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8 votes
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Why isn't twisted pair used for feedlines?

Balanced lines (of which twisted pair is a special type) really have an upper frequency limit; you can't use them to transport 1 GHz (well, you can, but the smallest variation in direction or distance ...
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8 votes
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Soldering PL-259.. with a torch?

Our friends in the UK and other parts of the world are now wondering how you could even begin to solder a PL259 connector with a torch (aka flashlight)! But in their vernacular, you of course are ...
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8 votes
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VHF 50 Ω Antenna Over 75 Ω TV Coax

I will need a UHF/VHF diplexer on either end to suitably merge/split the signals from each antenna Yes, this is correct. A tangent: If you wanted to save some money by using mass-market parts, ...
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8 votes
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50Ohm Coax presents 50 Ohms of impedance on which frequency?

For practical purposes, all frequencies. If you cut a transmission line into infinitesimal segments, each segment can be modeled as: simulate this circuit – Schematic created using CircuitLab ($...
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8 votes
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How to calculate coax cable loss for lengths other than 100 feet

It's the first one, if you use 50 feet (1/2 the length) then the loss is 20 dB / 2 = 10 dB. similarly, if you used 200 feet, the loss would be double… 20dB + 20dB = 40dB loss. there are several online ...
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  • 2,843
7 votes
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Does a balun need to be made with coax?

I'll explain the operation of that balun very briefly: for the differential mode (which by definition has equal but opposite currents on each conductor), each conductor induces an equal but opposite ...
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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 ...
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7 votes
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It it really useful to terminate unused 75Ω outputs on a coax splitter?

Terminating unused ports will never make things worse, and indeed is necessary to provide "ideal" behavior. Is ideal behavior really necessary? It depends on the application. Is your setup working now?...
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Can ferrite beads lower coax cable velocity factor?

No, adding ferrite beads to choke currents on the outside of the shield of a coaxial cable does not affect its impedance or velocity factor. Impedance and velocity factor are determined by the inside ...
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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 ...
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7 votes

What does an antenna analyzer tell me about these coaxial cables?

My name is Alex, I'm the head of technical support at RigExpert. This is actually an interesting question. Our engineers could not give an exact answer why the schedule in the second case behaves ...
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6 votes
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Weather-proofing antenna connections

What I have done once for a dipole was use a SO-239 panel receptacle with a solder cup to make my own "coax to two wire" adapter. For example, I used something like this part from Mouser. That way, I ...
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6 votes
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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. ...
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6 votes
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Suspected Damage in Coax Cable

It appears to be typical moisture ingress. The joint between the coax and the connector is not inherently waterproof. A damaged jacket can also be the source of moisture ingress. Normally the ...
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6 votes
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Why does ladder line generally have a lower loss than coaxial cable?

When the SWR is 1:1, the matched line loss of ordinary ladder line is lower than the matched line loss of ordinary coax because at HF, most of the loss is $I^2R$ loss, and the current magnitude is ...
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6 votes

Can ferrite beads lower coax cable velocity factor?

For well-designed coax, the EM fields are confined to the space between the inside of the braid and the center conductor, i.e. the dielectric insulation region which affects the velocity factor. ...
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6 votes
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Recomended coax for 100ft (2m/70cm)

According to this online coax loss calculator, the loss of 50 feet of new Belden 9258 RG-8X with an antenna with an SWR of 1.5 is 4.29 dB at 450 MHz (UHF), so the loss for 100 feet of the same coax ...
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6 votes

One cable two different radios

You need a switch. Connecting two transceivers directly to one antenna is a great way to destroy one of them — as soon as you transmit with one, the receive circuit of the other one will see several ...
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  • 8,740
6 votes

VNA basics, antenna coax cable

If you want to understand what your radio "sees", which is probably the case, then you want to measure the entire system including the cabling to the antenna rather than just the antenna ...
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6 votes
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VNA basics, antenna coax cable

You want to use the VNA against the final system configuration... or at least as much as practical. Why? The antenna's environment and transmission line will impact its characteristics. It's pretty ...
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  • 2,843
5 votes
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How is a 1:1 current balun different from a choke?

Baluns are designed to be transformers (like 1:1 4:1, 6:1, etc.) or choke baluns, and both. For an antenna, the purpose of a choke balun is to create a high-impedance to common mode currents that ...
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5 votes
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How to choose a coaxial cable

Your primary concerns are likely to contain: Characteristic impedance ($Z_0$). This is usually dictated by the other components in your system and is usually $50\Omega$ for amateur radio applications....
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5 votes
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Understanding coax radiation and "current on the shield"

Perhaps some of my confusion comes from an audio background, where one has "balanced" and "unbalanced" cabling in a way that is easier for me to understand: an unbalanced cable is simply a signal and ...
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5 votes
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RG-174 to pull signal from 1st IF?

So, you say: I don't care if the signals are delayed. I'm just tapping the line out to a pan adapter. I am most concerned about influencing the existing function of the receiver And that's exactly ...
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5 votes
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Use a TV coax and balun for a poor-man's OCFD?

OK, let me try to answer this, but this answer may also be qualified as unqualified. If you have a 50 Ohm receiver, and connect a perfectly (Z=R) 75 ohm antenna system, then your VSWR would be 1.5, ...
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