# Tag Info

18

Clearly capacitance is the key Capacitance is just one part of it. The gamma match in your question is three things: A sort of folded dipole, performing an impedance step-up A parallel shorted transmission line stub, adding shunt inductance A series capacitance An equivalent circuit is: simulate this circuit – Schematic created using CircuitLab So ...

13

A gamma-match serves a triple purpose: As a small diameter wire parallel and in close vicinity with the main radiating element, it will carry only a fraction of the main element current while being exposed to the same electrical field strength. This turns it in an effective up-transformer of the antenna input impedance. It also forms together with the main ...

10

So, by design, the elements of any Yagi have a zero current going through the center point. That's pretty obvious be symmetry: assuming you excite the "left and right" halves of the driving dipole with exactly opposing voltages, everything should be symmetrical across the plane through the middle of that dipole. Hence, if you approach that plane from left ...

8

Two Yagi antennas on the same antenna port could work for this application. Join the feed lines from the two antennas with a power divider for proper impedance matching. Make sure that they are mounted in-line with each other with the two driven elements some multiple of a whole wavelength apart. This ensures that the rearward radiation of one won't partly ...

7

It would be nice to think that there is simple formula or algorithm that one could plug the frequency and desired gain into, and out would come the required element spacings. The reality is that the math is difficult - requiring the solution of many simultaneous equations - and the results may still not absolutely mirror reality due to variations in ...

6

It's probably OK to use a different metal for the directors than the driven element, as long as you don't have dissimilar metals touching that would create a galvanic corrosion problem. But steel is a lousy conductor that would make your antenna less efficient. If you wanted to know exactly how much less efficient, then you could model the antenna in EZNEC ...

6

Depends on the particular Yagi or dish. Look for a published gain. Beware of low-quality manufacturers that publish erroneous data. Higher gain will correlate with higher probability of working. Point-to-point links of 1812 feet are easily accomplished, with a clear line of sight. I think you'll have 4 potential issues: Attenuation by the building will ...

6

You can make a first order approximation by using typical WiFi ranges in combination with two free space path loss calculations. 802.11b/g has a typical indoor range of 150 feet. By calculating the difference in FSPL between 150 feet and 1812 feet, the approximate antenna gain requirement can be determined. Using an on-line FSPL calculator, we find 73 dB vs ...

6

Yes, this is a time-honored practice, particularly at the longer wavelengths where horizontal antennas need to be mounted quite high to provide low-angle radiation for DX work. Callum, M0MCX, presents a nice gallery of information about his three-element parasitic array for 40-m: The radiating elements are the three black verticals in the foreground behind ...

6

The elements of a Yagi are dipoles. If you look at a dipole at any instant, the voltage at one tip will be some value, and the voltage at the other tip has the same magnitude, but opposite polarity. So let's say at some instant, the left side is at +50V and the right side is at -50V. Consider these relative to the soil at the base of the mast, if you like. ...

5

A few things to start off with: You don't care about efficiency, unless the antenna efficiency is truly abysmal, or your receiver is of very poor quality. Efficiency on the receive side is a measure of how effectively the antenna can couple with the ambient RF field and get that energy to the receiver. Even a receiver of very poor quality will have an ...

5

The MA5B is rated for 1,200 watts PEP. 200 watts is certainly well below that threshold. Things to check: Arcing, corrosion or moisture inside the match box A loose or corroded connection from the matchbox to the driven element Loose or corroded connections in the "X hats" A waterlogged, corroded or otherwise failed trap Loose or corroded connections in the ...

5

I suspect your results are an artifact of your optimization methodology. I simulated the design described in the link with NEC2 using the EZNEC front-end. For reference, EZNEC calculates the gain as 13.03-dBi in free space with 2-mm diameter elements (the link doesn't specify element diameter). I believe this is close enough to your model's gain of 13.6-dBi ...

4

I'm no expert on balun designs, but I think they mean to construct a folded balun, as follows. The coax's outer conductor is soldered to the same part of the antenna, but it is left on the coax, not peeled away, so the final balun structure includes a length of the coax. (The interpretation you proposed in your question is unlikely because if the outer ...

4

This is really a mixed question. It's not the mode that dictates the polarization. It's the situation in which that mode is used that dictates it. FM is typically only used at VHF and above frequencies where we have enough bandwidth to use it. VHF/UHF being line of sight is the choice for local work either simplex or via a repeater. For obvious reasons, ...

4

Merely trim the reflector element of one yagi to the length of the first director and it will become bi-directional enough to get the job done without the expense of another yagi and harness. Some retuning of the gama match may be needed

4

Mounting a vertically-polarized Yagi on a vertical conductive mast usually screws up the pattern. You either have to use a non-conductive mast, or offset them from it with a horizontal boom. 40 years ago, Cushcraft sold an 11 element beam designed for 146 MHz. Not realizing this, I (and countless other hams) mounted it on top of a steel mast. That caused so ...

4

You definitely don't want to mount a long metal pole through a Yagi. My rule of thumb is that it at best cuts off the Yagi at that point, perhaps worse. You have three options to avoid this. Mount the Yagi at its back, perhaps supporting the front with a non-conductive rope. The mast can get quite close to the reflector, 1/8 wave is ok. Offset it from the ...

4

Some general hints from my experience: Use wires not printed metal. You could stop the print for a while to drop in the wires, then finish it, or push them in later. For sanity's sake, design your yagi to use identical directors, and make or buy a wire cutting machine to produce lots of wires of the same length. Plastic will have an effect on tuning - the ...

3

Without all the technical banter take a look at G0KSC site on Yagi design. Justin explains it quite well... The bottom line is when using a non conductive boom or simply isolating the element's from a conductive boom the Yagi becomes much much quieter as far as the signal to noise floor goes. A matter of hearing the someone or not?? My conclusion from ...

3

I would bend the tips back, twisting the free end around the standing end so you make a loop with good electrical contact between them. Measure from the base to the end of the loop for the purposes of calculating the length. The loop is almost entirely cosmetic and won't affect the electrical length of the antenna much. If you were to make the loop bigger, ...

3

The easiest way is to simply connect them without any matching. You'll suffer an additional SWR of 1.5:1 due to the mismatch, but that's well within most transmitter's capability without damage. The second easiest is to simply employ a tuner. Neither method will give you the best output power, but they are certainly among the easiest methods of matching.

3

A Yagi-Uda antenna is a dipole antenna (sometimes folded dipole) with extra parasitic elements. Both the dipole and the parasitic elements are symmetrical; therefore the antenna is balanced; therefore it does not require any connection to ground to perform as designed. You may wish to use grounding for protection against lightning or electrostatic buildup, ...

3

If a full-sized VHF antenna is too big, then look at how people do direction finding on HF. A longer wavelengh means bigger antennas, so HF direction finders have been dealing with the "too big" problem for a long time. The canonical HF direction finding antenna is a small loop. This antenna is vertically polarized and gives two nulls when the plane of the ...

3

For diversity gain, you'd need to have two receivers (or you just switch to the currently "better" antenna; that's called selection combining in MIMO technology and kind of is the worst possible diversity mechanism).So, no, with two antennas and only a single receive chain, there's no diversity gain. You can, of course, build an antenna array. That would ...

3

In your post the thing that leaps out is about using a horizontally polarized Yagi to contact a repeater which is commonly a vertical antenna. The signal loss in this situation is large. Try your Yagi antenna with the elements pointing up and down and see which orientation better, horizontal or vertical. Does this make a big enough difference? As for ...

3

A complete analysis of this situation first must consider the propagation path performance, and then the types(s) of antennas to use w.r.t. the losses on that path. This short propagation path has nearly a fixed propagation loss regardless of the antennas/patterns in use. Antenna characteristics depend on path losses, the Z-matched power at the tx antenna ...

3

What you need is a link budget. A higher gain antenna (such as a Yagi) adds gain to your budget and makes the link more likely to work, or allows for less transmit power. I'm not sure what you mean by "oversizing" a link. Using an antenna with more gain than necessary has no particular disadvantage other than the antenna might be larger or more expensive ...

3

No real world antenna exhibits "zero" power in any particular direction. Careful design can minimize the power in the unwanted direction but not eliminate it. In a yagi antenna, the front to back ratio is the difference of the decibel gain of the forward, major lobe less the decibel gain of the lobe 180 degrees from the forward lobe. Thus a yagi antenna ...

3

Here are some hints : 1) two dipoles usually interact strongly, so the current on one is dependent on the current in the other. This is how a Yagi Uda antenna works - the elements are mostly parasitic, i.e. not driven by a source, but still carry current. In this case you don't have to worry about this at all. The two currents are given. 2) the fields at ...

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