Gap between a dipole antenna

Question

I need to know how much gap should be there between the two arms of a dipole antenna?

I would also like to know how much gap is there between the two rabbit ears antenna and is the loop antenna attached to the rabbit ears, is a folded dipole for UHF?

Like the one shown below.

https://www.rcaantennas.net/indoor-hd-antenna/?sku=ANT111Z

Answer 1

This question isn't as meaningful as you seem to think it is. There is absolutely no value that "the gap between a dipole" "should be". If a dipole is fed conventionally (directly connected to a transmission line or a TLT-type balun) then the "gap" between the elements is whatever the spacing is between the conductors of that transmission line. If you run wires out to some other spacing, those wires are part of the antenna, whether you think of them that way or not. For some other feed styles (isolation transformer, gamma or delta match), there's one continuous element with no "gap".

Answer 2

An ideal dipole has no gap at all between the two branches.

Reality is that this is technically impossible to realize, and gaps that are very small compared to the wavelength don't matter (enough to care about).

Also note that there's antenna types where the change between the characteristic impedance of the conducted wave on the waveguide (be it a ladder line, a coaxial cable, or something like a rectangular hollow waveguide) and the main emitting elements of a dipole is done exactly by tapering the conductor distance, to give a more wideband match than would be possible with a plain dipole, at the cost of not being as good as a dipole of the same dimension for the lower frequencies. It's trade-offs all the way!

Like the one shown below.

yeah, that's not really a classical dipole to begin with – it would only be if you bent both telescope conductors to lie flat, pointing in opposite directions. And then, that loop in front would still interact with the electromagnetic field, so under no circumstance can this be fully understood as dipole.

The design tradeoffs, probably in order of importance, for such an antenna like that are:

1. Marketability – antennas that look fancy are sold easier than antennas that don't, no matter whether the looks contribute to better antenna performance. And, boy, is that a design-intense three pieces of wire!
2. Wideband – you'll notice the antenna is sold for "FM and HDTV" reception. That covers the VHF low band TV and broadcast audio broadcast from 44 to 104 MHz, the VHF high-band between 184 and 216 MHz, the lower UHF band from 470 to 614 MHz and from 704 to 890 MHz. No actual dipole could serve all these bands at the same time – a dipole that works well for one frequency can't work well for twice that frequency. So, a proper dipole design is out of the question. Being a good antenna is not what you need to sell an indoor antenna – not being bad at any of these bands is
3. Adjustability – this ties in with marketability: because these antennas are inherently bad, and because they are used indoors, interaction with the multipath environment and nearby conductors is what makes and breaks the antenna performance. So, not only do you need to give the user the ability to adjust their antenna in a way such that their piping becomes part of the antenna (totally accidentally!), the older they are, the more likely they are to want that. Back in the day, fewer transmitters at lower frequencies meant that fiddling with wire or telescope antennas was what was needed to get good reception. Hard to explain in marketing material that for higher frequencies, a purpose-designed multi-band antenna, potentially paired with per-band selection combining, would by far outperform their hand-adjusted "living room artistry antenna". Remember that as antenna manufacturer, you don't live off the educational material on wave propagation and antenna theory tha you sell, but off people buying an antenna to be able to see their TV series or animal-named news broadcaster.

So, very little of the antenna and wave theory education in this world applies to the commercial antenna you show. Antenna theory can't justify mounting a loop directly next to two adjustable branches of some kind of "bent" dipole; the answer to your question hence is "the gap is as large as looks stylish, and still works when tested in a real living room".

Answer 3

The angle doesn't matter much. That isn't really a dipole antenna, but rather a related vee (or v-) antenna.

Modeling of an idealized vee antenna shows nearly identical behavior from flat to 30 degrees opening angle. Rotation is far more important, as you get a change of ~6 dB from the worst to best azimuthal rotation.

This is the benefit of vee antennas: you get space savings for little change in performance.

Yes, the vee/rabbit ears handle VHF, while the loop handles UHF. Again, rotation is important, as the power needs to go through the hole, and there's a sharp null when the loop is perpendicular to the direction of propagation.

There is going to be coupling between the two antennas, how much is hard to say without modeling the whole thing in a program like EZNEC.

Answer 4

It's a relic from the 50s and 60s from a company few folks remember—the good ole days of black and White TV. Rabbit Ear antennas have not changed much. The only improvement you can make is to place an LNA inside the base and use a USB plug for power.

Most Rabbit Ears use two antennas, telescoping dipoles for VHF and a loop for UHF. The telescoping dipoles are a good design. You can tune them if you know what you are doing and directional if horizontally polarized. You can configure the dipole as a "V" or conventionally. Conventional is the better of the two possibilities.

The distance between the feed points depends on the impedance of the transmission line. For example, Rabbit Ear antenna impedance is 75 Ohm and uses a 75-Ohm twin lead. The twin lead has a specific distance between the two conductors. The feed-point gap should match it. Higher impedance demands broader gap distances between the conductors. For frequencies lower than VHF, the gap is not an issue. If you look at amateurs' HF wire dipole antennas, they can have a several-inch gap.

The UHF loop is more of a gimmick than functional. However, consumers don't know any better. The only thing you can do with it is aim like the dipole. It is directional front-to-back. It works mediocre at one frequency. Chances are your UHF station is not using that frequency. You cannot tune it.