253 replies to this topic

[mtv]

Magnum72, on 17 September 2012 - 02:37 PM, said:

Cheers fellas-makes sence,
just another one to throw in the mix-mice/rats-yum yum cables!

Yep.... even tradies etc walking through the ceiling standing on and crushing poorly installed coax cables.

[vur]

Thankyou all for your attempts to help. I diligently tried all suggestions.

Frustratingly I have to fess up that under pressure from "the dreaded" I got an antenna guy in today.
The erratic reception of SBS was fixed by replacing a poor coax extension so that antenna continues to be used.

The antenna guy could not quickly work out why the buzz was occuring with the other antenna so on a service-cost basis we opted to ignore.

I regret that we could not learn anything from the exercise. Thankyou again.

[lekie]

Phased Array design question -  These have been around commercially for a long time and the design seems to unchanged as we enter the digital age (ie channels 6 -12)  Whether  2 or 4 driven elements (which on there own are just simple dipoles) are all the same length. For example on the Hills models are these half or full wave and what is the optimum frequency?  As they are not fold dipoles (300ohms) why are baluns used?

[alanh]

Leike
Phased arrays and Yagi-Udas have been around a very long time.

The typical phased array TV antenna has its dipoles a half wave length long in total length for each one. Reflectors are usually 10 % longer. This also applies to the Yagi-Uda Vertically the dipoles are connected with a quarter wavelength long harness, with the dipole/reflector a half wavelength spacing to the one above. At the junction of the quarter wavelength harness, the signal is available. Putting the dipoles in parallel reduces the impedance. The folded dipole acts as if it is a pair of dipoles. The quarter wavelength connection increases the impedance. Now the result is a 300 Ω output. In the latest Hills catalogue they are including a balun to go from 300 Ω balanced to 75 Ω.

The old Hills CA16 4 bay phased array was designed for channels 6 - 11 ie 174 - 222 MHz. The current use is from channels 6 - 12 which is 174 - 230 MHz.
The centre frequency for the old requirements is 198 MHz giving a half wavelength of 755 mm. The new requirements 202 MHz, 740 mm.

Alanh