Antenna Using a UHF antenna for VHF

kq6qv · Post by **kq6qv** » Thu Oct 28, 2004 2:46 pm

Some UHF antennas are advertised as working for VHF also. As the following net gain graph shows, there is not much truth in that. For channels 7-13, the MegaWave and the Winegard PR-8800 perform about as well as rabbit ears. The Square Shooter is terrible for all VHF channels. All of these antennas are useless for channels 2-6.

The surprise here is the Channel Master 4228, which has a lot of gain for VHF-high, especially channels 9-13. What makes it different is the screen that is continuous across all 8 dipoles. Other 8-bays, like the 8800 and the DB-8, have a reflector that is not continuous across the right and left halves, and thus they have no useful gain for VHF.
[img][592:500]http://www.hdtvprimer.com/ANTENNAS/GainVHFnetUHF.gif[/img]
The continuous screen is not simply a reflector for these channels, but rather it becomes the primary radiating element. As a result, radiation out the back is very strong, nearly bi-directional for some channels.
4228:
[img]http://www.hdtvprimer.com/ANTENNAS/CM4228Vaz.gif[/img]
If you buy this antenna for VHF there is one caution: The screen is in two halves that touch but are not solidly connected to each other. Even without a connection there is probably enough capacitance to complete the circuit. But for some units there might be some sensitivity to wind. (The author has not experimented with this and does not know how much of a problem it is. If he were using this antenna for VHF he would probably bind the two screens together more tightly.)

akirby · Post by **akirby** » Fri Oct 29, 2004 5:02 am

Ken - here's a question that came up on our local HDTV yahoo group.

Are DB8 and 4228 considered directional or multi-directional? What about their 4 bay counterparts?

kq6qv · Post by **kq6qv** » Fri Oct 29, 2004 2:22 pm

A 4-bay has a beam that is broad horizontally but narrow vertically. A 4-bay can be called a multi-directional antenna.

An 8-bay has a beam that is very narrow vertically and horizontally. Thus these two antennas are very different despite their apparent similarities. I don

donshan · Post by **donshan** » Sat Oct 30, 2004 9:30 pm

Ken has provided a useful solution for those with a channel 7 to 13 VHF station within range and needing UHF.

Just for a comparison I have the Channel Master specs on my model 3677 Crossfire combo VHF-UHF. This is the smallest one in the series and cost me about $52 plus shipping. Now working fine after Ken helped with a FM trap for an overload problem on my HR 10-250. This is the lower to upper range on gain, but the curve vs channel is not smooth. eg. the maximum gain may be in the middle of the channel range. The complete table is at
http://www.starkelectronic.com/cmg2.htm

channels 2-6 +1.5 to 3 db

channels 7-13 +7.2 to 9 db

channels 14-66 +7.8 to 10.2

I hope these are the same units!!! If not Ken please correct.

kq6qv · Post by **kq6qv** » Sun Oct 31, 2004 12:42 pm

The chart at starkelectronic.com says