Gray-Hoverman is an easy to build wideband UHF antenna. By adding some additional elements called NARODs you can also receive VHF band. The antenna offers sufficient gain for DVB-T, DVB-T2, ISDB-T and ATSC reception.
In the past, air TV channels were broadcast on frequencies ranging from 170 MHz to 860 MHz. The VHF-Hi band covers 175 to 230 MHz, while UHF covers 470 to 860 MHz. This is a large interval, covering two bands and it is difficult to receive with a single antenna. But, nowadays the UHF band for TV broadcasts is getting smaller. Frequencies starting from 600 MHz are being repacked for mobile networks.
The Gray-Hoverman antenna has been designed by Doyt R. Hoverman and it was patented in the sixties. It only covered the lower part of UHF, from 470 to 720 MHz (21 - 52 channels). If that could have been a problem, it's not anymore. The average gain in this frequency range is 13 dBi with reflector and 9.2 dBi without it. With some additional elements, this antenna can also receive the upper part of VHF from 170 to 230 MHz, with an average gain of 8.1 dBi with reflector and 6 dBi without it. The overall gain is not too much but, since it is easy to build and wideband it's worth a try. The original design has been republished and distributed according to the GNU GPL version 3 license.
The above is a 4nec2 3D render of the antenna including reflector elements. The top and bottom elements are called NARODs (not a reflector or director) and have been added to allow reception of VHF channels. The overall gain improvement when using reflectors is significant and, although the build gets a little more complex, I recommend you add them. Of course, you may test reception without reflectors and if you're fine with that, you may keep the antenna without them.
Gray Hoverman antenna with NARODs
Reflector elements are located 10 cm behind the active elements. There is an exception to this: the two reflectors for NARODs sit behind them at 29.2 cm. The antenna can be built with aluminum or copper wire or pipe of 4 to 8 mm diameter. It should be noted that the NAROD elements and their reflectors are needed only if you want to receive VHF band. Some people even built bending jigs, but depending on your skills it may not be necessary.
The gain of this antenna and its variants with or without reflectors and NARODs is shown below (4nec2 simulation).
The average impedance of this antenna in UHF is 260 ohms balanced. It's highly recommended to use a 4:1 wideband balun to connect the antenna to a standard 75 ohms coaxial cable. You can buy one or you can build one similar to those in commercially available antenna amplifiers. A good addition to this antenna is a wideband LNA. The popular LNA4ALL and SPF5189Z are cheap and easy to get. Note that the balun should be inserted between antenna and LNA, unless you get a balanced input LNA. Gray-Hoverman antenna should be mounted vertically. It is suitable for horizontal polarized signals.
Resources
- The plans of the original antenna designed by Doyt R. Hoverman on Digital Home.
- The site which published the design of the VHF NARODs is no longer available. A copy still remains on Wayback Machine.
- Instructables user unclesam built a bending jig for those NARODs according to the specifications found on the above link. Check his instructable.
Interesting.
ReplyDeleteSo far my experiments with the narods have determined the following:
ReplyDelete1) each narod is a half wave dipole antenna that can be tuned to a given VHF-Hi frequency using the standard math:
>> Full wave antenna length (m) = 300 / Freq (in MHz)
>> Half wave antenna length (m) = Full wave antenna length / 2
The complete length of each narod is what tunes it.
2) Each narod is coupled to the SBGH by sections that run parallel to, 1/2" away from, the topmost elements and bottommost elements of the SBGH.
3) The shape the narod is bent into doesn't appear to be relevant, as long the ends are straight and parallel to the topmost and bottommost SBGH elements (for coupling).
4) There is a price for the enhancement narods provide. If the narods are tuned low in the VHF-Hi band, then the high end of the VHF-Hi band will be lost (In fact, the antenna's directional reception is 'flipped' 180 degrees at, in the case, the high end of the VHF-Hi band. Net result: no reception.) Vice versa applies.
5) Technically, a center of VHF-Hi band tuning would be most prudent in the absence of the need to improve a specific channel. Band: 174-216 MHz. Center: 195 MHz. 300/195= 1.54 m Half-wave: .77 m = 30.28" narod length.
6) the author's given narod length of 34.5" tunes at the bottom of VHF-Hi, dropping off the top.
7) It should be possible to build two different length narods, tuned to mid-low and mid-high VHF-Hi band, and installed on the same SBGH, to smooth out the drop off problem for full band enhancement.
Dave
will this aerial work in new zealand.
ReplyDeleteAs long as there are broadcasts on common VHF and UHF TV frequencies, it should work.
DeleteIs this antenna uhf and vhf? is it possible for 100 miles?
ReplyDeleteYes, this is for VHF and UHF (see gain chart).
Delete