23

Indeed, why? A 5/8λ isn't resonant where a 1/4λ or 1/2λ would be. The reason is the radiation pattern. The pattern for a 1/4λ monopole is essentially a doughnut, tasty and a pretty good pattern especially for a VHF antenna. Extending the antenna changes the current distribution. This flattens out the pattern removing power from the useless (for VHF ...


14

You may be surprised to hear this, but the antenna you describe isn't actually a vertical, but a dipole. One half of the dipole is the IMax-2000, and the other half is your feedline. You can not simply not have radials and still have an antenna. An antenna works by making EM fields between two things. A dipole has two halves. A vertical works because the ...


13

The primary advantages of vertical antennas are that they are omnidirectional, and with an appropriate ground plane (radials) yield a low radiation angle; this reduces the number of "hops" that HF signals must make to reach their destination. Ignoring the ground plane, which might be radial wires or metallic screening buried just under the surface, vertical ...


9

A transformer can help and could also make things worse. I have some bad experience with this. The way that a radio is damaged by static is simple: Something charges up to a high voltage, and then there's a sudden breakdown resulting in a high voltage on the first transistor of the receiver. This can happen in a few ways: A long-wire or monopole type ...


8

Summary If you can have only one antenna and can chose between a vertical HF antenna or a relatively high horizontal HF antenna, go for the horizontal antenna. Modelling results In January 2008, L. B. Cebik, W4RNL (SK) published what was going to be the last entry of his seminal 10-10 News series. It turned out to be an interesting gain comparison of single ...


7

It doesn't really matter. You're asking about a topic called polarization, which is really important in VHF and UHF. It's the reason why rotating your HT can improve or harm your reception, and it's also the reason why some 3D glasses and sunglasses work. But because of the way HF propagation works, it doesn't really matter. Any ionospheric propagation gets ...


7

Cribbing a few quotes from answers to related questions, here's a start. From https://ham.stackexchange.com/a/195/1362: The primary advantages of vertical antennas are that they are omnidirectional, and with an appropriate ground plane (radials) yield a low radiation angle; this reduces the number of "hops" that HF signals must make to reach their ...


7

The purpose of the radials is to increase the effective ground conductivity, thus reducing losses. As you propose to have 16 radials but in four groups, I'd expect the efficiency to be somewhere between 4 radials and 16 radials evenly spaced. That said, if you need to do this to work around some obstacle, then by all means do it. You can also make the ...


6

Without having one of these antennas to disassemble (perhaps destructively), I can't tell you exactly how they are constructed. But maybe I can address some of your underlying concerns. Firstly, counterpoise. In one sense, this is an elevated screen of wires designed to take the place of Earth. This sense developed with the Marconi antenna (what we'd ...


6

In your particular case, I'd note 2 things: The aluminium is directly above the whip, where the antenna's fields are the weakest. Aluminium is a good conductor, so unless there is something about it that will increases losses, it won't decrease the radiated energy. It might change the feedpoint impedance, and it might change the direction in which energy is ...


6

Perhaps consider that the objective of the ground plane is to present a low impedance. At the feedpoint, the hope is to have all the current go into the antenna, and none of it on the coax common-mode. The lower the impedance of the ground plane, the less current will be on the coax common-mode due to its relatively high impedance. If the ground plane ...


6

I congratulate you on your interest in working out the design issues from the ground up. Understanding the theoretical and comparing this to the field results is the beginning of a life long enjoyment of antenna experimentation. The Antenna By way of background, a 5/8 wave antenna is the highest directivity, single element, linear antenna that you can ...


6

The number of horizontal radial wires used in an elevated "counterpoise" can be far fewer than the 120 buried wires used by most AM broadcasters, and the even fewer numbers of buried wires often used with vertical monopoles, inverted Ls etc by amateur radio operators. Several AM broadcast stations use 3 or 4 pairs of λ/4, co-linear, horizontal radial wires ...


6

Rule of thumb: if the radials are elevated, at least two resonant radials for each band. If the radials are buried or lying on the surface, at least 16 radials, each at least a 1/4 wavelength at the lowest operating frequency, and don't worry about resonance: just get as much wire in the ground as you can. More and/or longer is better. The objective of any ...


6

First: do you know about pskreporter? You're definitely getting heard, including one spot on 40m from New Zealand. Sometimes FT8 takes some patience, and it seems like no one wants to come back to you for long periods of time, but pskreporter can at least provide some reassurance that your equipment and the ionosphere are actually in working order. That ...


5

To begin the discussion, it is helpful to understand the effects of shortening any antenna to a length below resonance. In all cases, the directivity of the antenna is reduced but this tends to be a fairly uniform reduction regardless of how much shortening occurs. The reduction in length also reduces the radiation resistance of the antenna. This is ...


5

Knowing that 1/4λ verticals are known in VHF/UHF applications to have nearly isotropic patterns This is not a valid assertion. A UHF/VHF vertical antenna has > 0 dBi gain (more succinctly a directivity >1) so by definition it cannot have nearly an isotropic pattern. Its primary directivity is toward the horizon at about a 22 degree elevation (although ...


5

A common belief is that Earth conductivity mostly affects the "takeoff angle" of a vertical monopole. However the NEC4.2 study below shows that the shape of the radiation patterns is quite similar when the same monopole system is installed at sites where Earth conductivity differs through the range from poor to excellent. The biggest effect when going from ...


5

Here is an analysis of a vertical, 1/2 wavelength, 2 meter dipole suspended at various heights above ground: As you can see, as you raise the bottom end of the antenna from near ground level up to about 7 wavelengths, the gain increases. At the same time, the elevation angle of the maximum gain decreases (becomes closer to the horizon). As you continue to ...


5

Almost any 160 meter antenna is a compromise. With limited space or budget, the compromise tends to be larger. At first glance, a dipole that is shortened by loading coils might appear quite attractive. With well designed coils, the efficiency of a shortened dipole can be relatively good and the gain can be within a few dB of a full length dipole. But ...


5

Elevated radials are required in order to increase/optimise the radiation possible from any such elevated "GP" monopole or whip antenna. If the elevated radials are symmetrically arranged and lie in the horizontal plane, then their net far-field radiation is zero. However the r-f current at the common point of those radials is not zero — it is the same as ...


5

Sure, that can work. Here's a monopole yagi I designed: Photo: Alaris Antennas MONO-A0005. It's a half-yagi for remote control of a mine locomotive, improving the control range by firing one way down the tunnel. The driven element is a half-folded-dipole, a good match for 50 Ohms. Also, it's welded up from solid 20 mm steel so it survives low ceilings, ...


5

It may very well be that the open contacts are arcing, and I'm betting that's what is happening. Here are some possible solutions. How about using different relays that have less tendency to arc: Open-frame relays with much wider contact spacing Vacuum relays Multiple relays in series Another possibility might be that RF on the control lines may be ...


5

Assuming that power losses in the transformer are acceptable, and that it's placed after 1:1 current balun (which eliminates common-mode current), is there any reason why it might be a poor solution? Well, the assumption about loss may not be a great assumption to make. It takes a substantial core to avoid saturation and overheating even at modest transmit ...


5

The purpose of the ground radials isn’t to ground the counterpoise. The purpose of a dense field of radials is to emulate as much as possible a perfect mirror surface, so that, to the far field, the vertical monopole seems to have a mirror image half that makes the monopole seem more like a full size vertical dipole, with a pattern maxima orthogonal to the ...


5

If you don't route the feed line away from a center-fed dipole antenna at 90 degrees, it will be in an unbalanced RF field, and thus will couple to the unbalanced RF field, to some degree, even with a perfect (infinite impedance) balun. Depending on the distance and angle of feed, an RF coupled feed line can act like an angled antenna director or reflector ...


4

An angled full-wave delta loop can be hung from the tower at the height you are talking about and is very broadbanded (and can be fed from a corner so that it is vertically polarized. An advantage is that this also makes a very good receive antenna. See for example the October 1984 QST for an article on the subject.


4

What are the rules of thumb for getting ground plane dimensions for a mobile antenna beyond "the bigger, the better"? There isn't one. An monopole antenna does not reach ideal performance until the ground plane is infinite, and the ground plane can be arbitrarily reduced in size and it will still work in theory (although at some point in practice, the ...


4

If you were to flip the antenna upside-down, ideally with no part of the antenna touching the ground, then your antenna radiation pattern wouldn't change all that much, as you've discovered on EZNEC. Two things have changed compared to a 'normal' ground-mounted vertical: you've elevated the radials, and you've flipped the antenna. (The following ...


4

The effects of using unequal length buried radials on the radiation patterns of a monopole are not very significant, as shown by the NEC4.2 analysis below.


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