Hi Sean,

Thanks for posting the picture of your antenna. I assume from your picture you actually have yours outside. To answer your questions:

The two most important considerations in antenna design are the radiation pattern and the input impedance of the antenna.

The pattern is important for efficient use of your energy. For a satellite in space you want a very directive beam aimed at a particular spot on earth. You donâ€™t want to waste your energy radiating into space. For a TV or radio transmitter, in general, you want to radiate energy everywhere in all directions in the horizontal plane, but not directly up in space. (Also, an antenna â€œbehavesâ€ similarly when it is transmitting or receiving, and often times, it is easier to visualize and describe things in terms of transmission instead of reception, which I will probably do as I write this).

Just as important to good antenna performance is the IMPEDANCE of the antenna. And impedance is usually the harder parameter to design for. To deliver maximum energy to a load, the impedance of the load must match the impedance of the transmission line. If you are using 300 Ohm twin lead for example, you want the impedance of your antenna to also be 300 Ohm. Any other impedance will result in energy reflected back due to the impedance mismatch. For example, connecting a 300 Ohm load to a 75 Ohm transmission line results in 36% of the energy being reflected, and only 64% utilized.

(One aside hereâ€¦ Twin lead wire has â€œbalanced currentsâ€ on its conductors. Co-ax cables have unbalanced currents. The outer conductor is grounded and the inner conductor carries all the current. So the device that transforms a 300 Ohm twin lead to 75 Ohm coax does 2 things. It transforms from BAlanced current to UNBalanced current, hence the name balun, and it transforms the impedance from 300 Ohms to 75 Ohms).

*Edit: Evidently, these baluns don't usually bother to match the impedance, so there's a significant insertion loss when using these.*
Anyway back to subject matter, if you are operating at a single frequency (or extremely narrow bandwidth), there are many simple techniques for matching the impedance. But if you are operating over a wide bandwidth, such as the entire VHF to UHF TV band, that is not so easy.

UHF TV broadcast is horizontally polarized. The simplest antenna to use would be a thin wire dipole oriented horizontal, or parallel to the earth. Thin wire dipoles are however very narrow band from an impedance consideration and are generally used at their â€œresonantâ€ length of slightly below half wavelength (about .47 wavelength). But the UHF TV band is almost an octave in bandwidth (470 MHz to 806 MHz). So if you use a thin wire dipole, it would work great at whatever frequency the dipole length is half a wavelength of, but get too far from that frequency, and your impedance mismatch losses will kill you: you would not be able to receive or transmit energy. But you can increase the bandwidth of your dipole by making it â€œfatterâ€ than a thin wire dipole, and by the proper shaping of itâ€¦ such as using a ..... bowtie shape. So a bowtie antenna is just a fancy dipole, and the particular bowtie with fin shape used in the RS antenna was probably done to achieve an impedance that is flat (or at least easily matchable) over the entire UHF band.

Without going into all the subtleties of array theory, 2 identical antennas connected in parallel, equidistant and equiphased from your source would result in double the power received. (As a quick side note, when you connect two identical antennas up in parallel, the net terminal impedance is split in two just as in regular circuits. Additionally, two antennas close to each other have a mutual impedance effect, i.e. the impedance of an antenna can be changed by having another antenna close to it, and all that has to be taken into consideration in designing your antenna system.)

With regards to the metal grate behind the antennas, it is simply a reflecting ground plane: essentially a mirror behind your antenna. The shortest UHF wavelength is 14 inches. As long as the grate spacing is small compared to your wavelength, it will appear more or less like a solid conductor â€“ like bouncing a basketball off a chainlink fence.. (Itâ€™s cheaper and lighter for a grating instead of solid piece of metal). In theory, with the right spacing of the antenna off a sufficiently large ground plane, you can conceivably double the received power in one direction, at the expense of the other direction.

While weâ€™re at itâ€¦ letâ€™s talk a little bit about gain and directivity. Both those terms are generally expressed quantitatively in dB. In general, when you are talking about dBâ€™s, you are talking about a ratio of one quantity to another. In many cases, there is some standard reference quantity that is never mentioned. For antennas, it is usually an ideal isotropic antenna (one that radiates equally in all directions of 3 dimensional space). For the sake of describing it easier, imagine a dipole as the axis of earth. A dipole does not radiate (or receive) along its axis, i.e. in the directions of the north and south poles. If you fed the same amount of power to a matched isotropic antenna and a matched dipole, the dipole will transmit more power along the equator compared to the isotropic antenna (conservation of powerâ€¦ if no power is going to the poles, more has to go to other directions). The amount of maximum power ratio COMPARE TO AN ISOPTROPIC ANTENNA is known as the directivity (or directive gain) or an antenna. For a short dipole, the directivity is 1.5 (or 1.76 dB) and for a half-wave dipole, it is 1.64 (or 2.15 dB). You sometimes see it written in dBâ€™s as dBi to explicitly indicate that the reference is an isotropic antenna, but most times it is just written as dBâ€™s with the reference to isotropic implicit.

What GAIN is, then, is simply DIRECTIVITY with all losses taken into account, such as resistive losses, impedance mismatch losses, polarization losses, etc. i.e. real world considerations. So for the RS double bowtie antenna: a single bowtie would have a directivity of approximately 2 dB, rounding to the nearest integer. (I think most people are aware that dB is defined as 10*log(power ratio) and twice the power is a 3 dB increase and 10 times the power is a 10 dB increase). Using 2 dipoles, you would increase your maximum directivity by 3 dB, and having a ground plane increases by a maximum of another 3 dB. So your directivity would be at best 8 dB. But across the entire UHF band, you will have impedance mismatch losses, also the metal grating is not a perfect ground plane, and is finite in size, so you wouldnâ€™t have exact 3 dB due to your ground plane. If the match is decent over the entire band, you might end up losses of 1 to 3 dB, so your antenna would probably have a net GAIN of 5 to 7 dB depending on where you are on the UHF band. This is a quantity of how much better you are doing in the direction of maximum radiation (and reception) compared to using an isotropic antenna. And if you look up the

specs of this antenna on Radio Shackâ€™s website , thatâ€™s exactly what they spec this antenna to: â€œApprox. 5 ~ 7 dB â€

Sorry this is so long, but I hope itâ€™s clear and some people find this information useful.