27

An excellent question! Without diving too deep into the theory, let's start with a few basic terms. The "signal" that an antenna is receiving or transmitting is called an electromagnetic wave. This is exactly the same type of wave as light. It is just that our eyes are sensitive to a narrow range of frequencies that we call light. Electromagnetic waves ...


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 ...


11

Having Ethernet and your antenna co-located is not an ideal situation. But then most amateur antenna situations involve compromises. The general idea of the following recommendations is to take as many precautions as practical to minimize the interference possibilities. I recommend that your Ethernet cable to your WAP be a CAT6 shielded cable (STP). While ...


10

And if it is possible to use a tuner to make a transmitter feed power into a coat hanger then why do we worry about making resonant antennas in the first place? In a word - efficiency. Consider that a full size 40 meter, 1/2 wavelength dipole is approximately 65 feet (19.8 meters) long. A dipole that is only 2 feet (0.6 meters) long, would have a gain of ...


9

As one solution, you can combine the antennas with a power divider. See How to combine two 50 Ω antennas such that they appear as one 50 Ω load? This makes your pair of antennas into a phased array. If there's no overlap between their coverage, you effectively lose half your antenna gain, or 3 dB. This is because on transmit, half your power goes into the ...


9

Antennas are often resonant. Their physical dimensions are adjusted so standing waves develop at a particular frequency, like a bell rings at a particular tone. Feedlines are not usually resonant. Usually an engineer ensures the end of the feedline is terminated (by the antenna or the radio) with an impedance that matches the characteristic impedance of the ...


8

Back in the olden days, before transceivers took over, the transmitter and receiver were separate units, and the receiver had to be protected from the transmitter. The solution then, as now, is an electronic T/R switch. One approach is an RF sensor that triggers a relay; the MFJ-1708 is one example. A more sophisticated approach uses an active element to ...


8

Yes, this works fine. It is often called a "fan dipole". There are separate resonant dipoles for each band, all fed from a common point. The wires should be separated by a foot or more. If you have enough tie points at the ends, you can run them in slightly different directions. The resonant dipole will have a low impedance, around 50 Ohms. The non-...


8

As resistance is to DC circuit analysis, impedance is to AC (or RF) circuit analysis. Now let's take a closer look at what that means, from several angles. I'm going to not include any of the math, and just state without proof the various concepts' relationships. In simple, idealized circuit analysis of DC or digital circuits, we often assume that lots of ...


8

The basic concept is to use RF bandpass filters for each frequency range. This is frequently done by hams for VHF and UHF applications. The common term for a grouping of these filters is diplexer, triplexer, or quadplexer as appropriate. Sometimes the term "duplexer" is used although this creates confusion with a different device that is typically much more ...


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 ...


7

"Ground" is not the same thing as "Earth". It may be confusing because the surface of the Earth is sometimes called "the ground", but still this is an entirely different thing than electrical "ground". It is often convenient to define ground to be Earth, but that's no requirement. In a battery powered system far away from Earth, "ground" is likely the ...


7

Your solution to "RF in the shack" should be proper antenna design first, and grounding second. See Using a balun with a resonant dipole (or any other antenna, really). If you take care to address common-mode currents, you won't need a ground. Regarding lightning protection, you might just forget about it. If lightning has struck your indoor antenna, it's ...


7

Yes, you can do this. There are two issues: connecting it to the radio, and the effect on reception. The latter might be problematic enough that using two receivers is actually a better option. Connecting two antennas to the radio If you simply connect two antennas in parallel, the math is just like connecting two resistors in parallel: the impedance is ...


7

My understanding is that they can neutralize the imbalance between inductive and capacitive reactance so the transceiver can couple the most power out (or in) even though an antenna might not be resonant on a particular operating frequency, since overall impedance (Z) consists of three parts, two of which are reactance dependent on frequency: X(sub)L and X(...


7

A wideband antenna is not what you're looking for – you really don't care about anything between 900 and 1500 MHz, or between 1600 and 2400 MHz. Wideband antennas are inherently hard to make, and even harder, even impossible, to make uniformly good across their whole range. What you much likely will rather want is a multi-band antenna. For example, I'd ...


6

1% here is just an overly precise way of saying "really small compared to the wavelength". The problem is that when the conductors are close together, their electrical and magnetic fields cancel; as they get farther apart, there's a larger volume around the line where the fields don't cancel, and the ladder line starts looking more like an antenna than a ...


6

That statement is wrong on several levels. The antenna is DC grounded so no lighting arrestor is needed. A lightning arrester is needed, even if the antenna is DC ground. The arrester's job is to limit the center conductor's voltage to be not very different from the shield. That the antenna is "DC grounded" isn't worth much. Lightning is not DC. In fact,...


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

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 – Schematic created using CircuitLab A common-mode choke works in either direction, so it matters not which end is balanced and which is unbalanced (or if both ...


6

The short answer is that it can't. A shielded transmitter, connected to an ideal piece of coax, does not generate common mode currents. The inner and the outer of the cable look connected (and for Direct Current they are), but high frequency currents really cannot pass through the thick metal, they are confined to the inside or the outside. Real coax can ...


5

Your attic is pretty low but not quite low enough to be a problem for installing a vertically polarized 144 MHz antenna. There are two common antenna designs that would suit your case: Ground plane antenna. This simply consists of a vertical quarter-wavelength element (stiff wire or rod) sticking up from a horizontal ground plane (sheet metal, as big as ...


5

Rather than two receivers, maybe a switch that selects between antennas could be a cheaper option?


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

I'm not at all sure about the exact situation, but I predict that you will find that the cable between the distribution amplifier and this particular TV set is damaged. In general, when you omit the shield connection to a coaxial cable, it starts operating more like an unshielded single wire — a monopole antenna, if the other end is disconnected. For ...


5

You may be overloading the front end of the "faulty" TV, creating adjacent channel interference or some other kind of distortion. The loss you introduce by only loosely coupling the receiver may be bringing down the signal level to a point where it is no longer impairing the receiver. The fact that a distribution amp is inline with your television probably ...


5

Congratulations on getting permission to install an antenna. There are many hams that rent that would love to be in your position. As Phil correctly asks, why do you need an earth connection? Here are some possible reasons to be thinking about earthing in your situation. Common Mode Current An OCF antenna is an inherently imbalanced antenna. Such an ...


5

...variable caps are ... no longer manufactured in the old "interleaved plates, air spaced" form, and the tiny plastic dielectric ones that are still available can't take much voltage (and are difficult to adjust precisely). Thankfully, Oren Elliott is a surprisingly affordable source of brand new air-variable capacitors. I have used them successfully in ...


5

Fair-Rite makes ferrite rods which would be suitable for HF applications. It should be possible to create or repurpose a screw-operated mechanism to move the rod into and out of a cylindrical coil. Preferably, the material will have steady permeability and low loss over the frequency range of interest. Loss is proportional to the ratio of the real and ...


5

If your transmitter emits enough power to be heard more than a few meters away, the likely result will be destruction of components in the receiver and possibly in the transmitter as well. Receivers are designed to amplify, detect, and decode signals in the range of a few to a few tens of microvolts; transmitters as weak as a few watts will put tens of ...


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