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 ...


12

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 systems when located at the feed point of the antenna. The only disadvantage is the extra loss in the coax from the extra length. Loss in a coaxial cable, when ...


11

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, dielectric losses, etc. and are specified in dB per unit length in the coax datasheet. At VHF and up, these losses can be significant even at car lengths, especially ...


10

"RG" stems from old, obsolete military specifications, standing for "radio guide", and the number is actually arbitrary. Part of the reason they are obsolete is that they weren't specific enough - RG-6 can reasonably be assumed to have an 18 gauge solid center conductor, but beyond that the dielectric, velocity factor, or completeness of the shielding ...


8

Executive Summary Assuming: air dielectric (insulator) 50Ω characteristic impedance Then for round coax, make the inside diameter of the outside conductor 2.302 times larger than the diameter of the inside conductor. If the shield is square, and the inner conductor is still round, make the inside length of one side of the shield 2.134 times larger than ...


8

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 would have catastrophic effects, and the conductor distance would get pretty small). This can be seen in technical practice: 100 Mbit/s Ethernet (Fast ...


8

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 referring to a burning torch. In general, when you heat a metallic object with the hopes of applying solder, the heat will cause oxidation to form on the metallic ...


8

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, you could use 75 Ω power dividers (coax splitters) instead of diplexers. This has 3 dB loss because the signals are not directed exclusively to the intended ...


8

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/Velocity_factor


7

The most important thing about the cable is how much loss are you willing to accept in the feed line. Figure out how much cable you will need, and then determine what the loss on said cable will be. As I mentioned on my website, here's a few good rules of thumb: Keep the power loss to no more than 3 db in the cable. If you can, use the same impedance as ...


7

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 antenna, making it appear electrically longer than it is physically. Without the inductance, the antenna would need to be approximately a quarter-wavelength ...


6

It looks like you're looking for an intuitive, practical understanding rather than precise definitions, so I'll see what I can do with that, with my own recent learning. The reason you care about impedance matching is that impedance mismatches cause the signal to be partially reflected — some of the energy is going the opposite direction than you want it to ...


6

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 had a whole piece of coax running to the antenna that could be weatherproofed, and then an exposed section that just had regular wire soldered to the receptacle....


6

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 magnetic flux in the core. These magnetic fluxes cancel, and so the differential mode sees no inductance: it's as if the core and the windings aren't there. ...


6

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. Since the SWR limit is the result of losses in the feedline, the efficacy of the 'high SWR fix' must be considered. The mechanism has to do with the trips that ...


6

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 connector to coax joint must be covered with a self sealing adhesive tape or heat shrink tubing with an interior thermal adhesive in order to protect it from the ...


6

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 inversely proportional to the characteristic impedance of the feedline. Of course, a high SWR results in higher standing wave currents and thus increases the $I^2R$...


6

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 construction of the cable: the outside diameter of the inner conductor, d, the inside diameter of the outer conductor (shield), D, and the magnetic permeability,...


6

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. Therefore, those beads have negligible effect on the differential signals. They do have an effect on the common-mode signals on the outside of the braid which has ...


5

I'd recommend one of two solutions: 1. Don't throw it out the window. Most lightning damage comes not from direct strikes but nearby strikes, which can still induce large-ish voltages on the feedline: not enough to make lightning, but enough to damage things. A couple feet of air between the feedline and the tranciever will protect against this. If you do ...


5

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 what you're going to do with this. Your Opamp circuit has a high impedance, which translates into "looks like an open end". That means that your tapping coax ...


5

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, and the "load mismatch attenuation" will be about 0.177dB. (with antenna system I include feedline) I doubt that you would actually notice this. However, you ...


5

Based on your description, I would suspect moisture damage. Moisture ingress in coax cables typically results in the corrosion of the copper braided wires in the shield. The oxide that forms increases the RF losses of the shield. A simple resistive test of the shield is not usually sufficient to detect this condition. If the center conductor is multi-...


5

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? If so, terminators aren't necessary :-) A typical splitter has these properties: It has an input port and two output ports, which we'll call A and B. It has ...


5

RG-174 cable does not offer very good shielding. This can be a significant problem when you are attempting relatively high levels of attenuation as the cable leakage may provide more signal than the attenuator is allowing. Here is an illustration of coaxial cable leakage from Wikipedia: There will be losses in the "transport mode" that are due to the RF ...


5

I've often used a propane torch to quickly preheat PL259s, but seldom for the actual soldering. Once the connector is preheated, it is a quick and simple matter to flow solder into the holes and the pre-tinned braid using a 50 watt soldering iron. (These days I use a Steinl heat gun instead of the torch flame for preheating, as it gives much better control ...


5

Glad to see this question posted! This has a really interesting answer which I'm not qualified to write a comprehensive answer to, so here's a rather unsupported answer to help you along until someone writes a better one. There are several reasons why coaxial cable is used in several different impedances. On basic principles: if a device on one end of the ...


5

RF Cafe cites quotes Harmon Banning of W.L. Gore & Associates, Inc.: In the early days of microwaves, around World War II, impedances were chosen depending on the application. For maximum power handling, somewhere between 30 and 44 Ω was used. On the other hand, lowest attenuation for an air filled line was around 93 Ω. In those days, there ...


5

You should use a four-port multiplexer, more commonly known as a triplexer: As shown in the diagram, a triplexer is a bidirectional device comprising three filters that separate (or combine) the signals from the input (or output) port into band-limited signals at the three output (or input) ports. A properly designed triplexer will present 50$\Omega$ at all ...


5

You have mixed up the characteristic impedance of a coaxial cable with the DC impedance measured from a piece of that cable. So I'll tell you what you measured and then what you could have done instead. First of all, I have no idea what does it mean to measure a capacitance with a multimeter when you have connected both measurement leads to the same piece ...


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