What would happen if you made an antenna consisting of two perpendicular dipoles tuned for the same frequency and connected to the same feed line? Would this work to avoid polarization mismatch loss?
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1Circular polarization can sometimes improve the match over a crossed polarization. One way to do a circular polarization is to have dipoles at 90 degrees...but they have to also be phased correctly. Either not coplanar or with a phasing harness between them. – user10489 Jan 03 '23 at 05:31
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1The thing you're describing is called a [Turnstile Antenna](https://en.wikipedia.org//wiki/Turnstile_antenna) – tomnexus Jan 05 '23 at 17:26
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1Turnstile antenna is one of the most popular variations of this pattern. – user10489 Jan 07 '23 at 05:54
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Any combination of two antennas into a single feed line is simply a more complex antenna, which will exhibit a single polarization — see the other answer by hobbs for illustrations.
However, there is a way to avoid polarization loss using multiple antenna elements — antenna diversity. Instead of passively combining the two antennas' signals into a single feed line, you need some technique that adapts to the signal conditions, such as:
- For your ham station, perhaps a manual antenna switch you set to whichever antenna is currently working better.
- Two feed lines and two receivers, which could work like...
- Listen to separate versions of the signals in each ear. (Some amateur transceivers are capable of this in one box.)
- Receiving digital packets: run two decoders and discard any duplicate packets.
- Receive any signal better this way with the right DSP to compare the signals' phase and sum them.
- A variable phase shifter on the second antenna, to make a phased array antenna, which can be adjusted to exhibit any polarization.
This type of thing — multiple antennas, though not necessarily antennas in crossed-dipole configuration — is routine practice in modern digital communications such as Wi-Fi and the cellular network, to ensure that connectivity is maintained regardless of the orientation and position of the mobile end of the link.
Kevin Reid AG6YO
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2The simplest version of the last point, for hams, is two antennas, and two receivers (often in the same box), feeding left/right speakers or left/right headphones, letting your brain automatically "tune in" to the stronger/clearer signal. – hobbs - KC2G Jan 02 '23 at 19:12
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1@hobbs-KC2G Thanks — I knew that was a thing but forgot when I was writing the answer. Edited. – Kevin Reid AG6YO Jan 03 '23 at 16:06
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When I worked on (management software for) commercial fixed radio transceivers, one of the recommended configurations was to mount an extra receiver on the same pole, facing the same way, but many wavelengths below the main one. "Fading" caused randomly by objects moving within the link's Fresnel zones would be unlikely to affect both receivers equally, and (the DSP team) had some algorithm that would recover the data from either receivers. – user253751 Jan 06 '23 at 00:01
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@user253751 That sounds like a doplper effect correction for mobile stations moving away from the antenna to avoid "Picket Fencing" – Jan 13 '23 at 04:24
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1@Strom No - fixed link, but metal objects between the ends of the link (like cars) move around and cause random multipath. At certain unlucky moments, the multipath interference exactly destructively cancels. – user253751 Jan 13 '23 at 07:55
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No. This:
equals this:
which equals this:
except that it has somewhat more bandwidth and a different impedance because of the "fat" elements. In other words, all you get is a dipole with a polarization "halfway between" the two dipoles that you crossed.
On the other hand this:
doesn't radiate at all, because elements of the same polarity are opposed to each other.
hobbs - KC2G
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1Actually, the triangular version isn't quite the same as the dipole.... it has better bandwidth. – user10489 Jan 03 '23 at 01:43
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@Strom true enough, but elliptical polarization is just another polarization, and any given elliptically-polarized antenna still has a corresponding orthogonally-polarized wave that will exhibit 100% loss. So with a single feedpoint and an antenna that isn't dynamically changing its geometry, you will always have fade. – hobbs - KC2G Jan 17 '23 at 08:04
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1@Strom you're mistaken. Any two polarizations that are opposite each other on the Poincare sphere are orthogonal. H/V and LH/RH make up two *canonical bases* which you can add together to get any elliptical polarization, but for any polarization if you invert the handedness *and* rotate the linear component 90 degrees you get an orthogonal polarization. It's just that for linear polarizations "invert the handedness" does nothing, and for circular polarizations "rotate the linear component" does nothing, which makes them simpler. – hobbs - KC2G Jan 19 '23 at 01:48
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1@Strom (or if you want to deal with it as a "mix" of pure linear and circular states, the point still remains... for an elliptically-polarized antenna, there is *some* elliptically-polarized wave such that the sum of the response to the circular component and the response to the linear component is zero. Yes, of course, any slight variation from that results in not-quite-zero, but that's [another question](https://ham.stackexchange.com/questions/21575/how-can-cross-polarization-loss-be-determined-given-the-angle-between-two-antenn)). – hobbs - KC2G Jan 19 '23 at 01:51
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"Any two [opposite] polarizations ... sphere are orthogonal." that is absolutely true(you have just stated orientation invariance, or in my world a matrix multiply to give a new system basis ), in theory. Take any 2 meter FM handheld and rotate it 90 degrees to horizontal, given you are close enough, you will still get the signal(at a major loss). – Jan 19 '23 at 02:18
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I am not mistaken. At no point does any true "Zero" exist in a real environment. In theory yes. Just like the infinite impedance half-wave antenna is not possible, since theory does not fit reality. – Jan 19 '23 at 02:26
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1Let us [continue this discussion in chat](https://chat.stackexchange.com/rooms/142168/discussion-between-strom-and-hobbs-kc2g). – Jan 19 '23 at 02:57
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Your last example is wrong. It will still, radiate perpendicular to the plane, straight up and down. the ground reflection could boost or cancel, depending on height wavelenths to to real the ground plane. Introduce an artificial ground plane and this becomes an EME, or Meteor, or NVIS antenna, if you angle the ground plane for aiming. – Jan 19 '23 at 05:07
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Yes, and no.
Assuming the dipoles are oriented one vertical and one horizontal with respect to level ground. Each antenna separately connected would receive both components independently. This is the reason high-end receivers have multiple active inputs.
This technique uses separate but synchronized receivers and reduces fading caused by changing atmospheric conditions. Fades on the left channel and grows on the right one.
The second you combine the feed lines from the two antennas half of the power coming in is reflected to the other antenna transmitted and mixes in the near field of the both antennas causing an elliptical pattern to emerge this pattern can be either constructive resulting in gain or destructive resulting in loss depending on the polarity of the incoming signal, and the phase of the signals at the join point.
To make this an effective solution, you would need to vary the phase based on the incoming signal. Make a device to variably delay the phase, this could be a simple as a double rotational switch to connect various lengths of wire to one of the feed-lines or a locked variable capacitor/inductor pair to introduce various delays without affecting the reactance.
Is it possible to make this work yes, but the manual adjustments required during fading events are only beneficial during receive; on transmit, there is no feedback to make adjustments. It would be slightly better,or worse(+- 3db) on transmit than a single polarity antenna.
If you want to invest the time and money, go for it. It may not be easy, but it does sound fun.
Minimizing fading is not the only reason for this configuration to be successful, the atmosphere elliptically polarizes signals, depending on direction, the amount varies, but polarization direction is consistent east to west(this effect is less consistent nearer to the poles).
In other words, you can target your signal to the polarization of the target dx stations antenna by counteracting/extending the atmospheric conditions. Conditions do vary over time, but the overall pattern does not.