hi again
I havent had time to try it yet mainly because it has been
wet but i have been do some futher enquiries over here and here are some relys i received
1- The higher the gain of both antennas, the better it works. Works great on 10GHz with two 50 db gain dishes.
If you want to amplify the signal, then you can't have the gain of you amplifier exceeding the effective front to back ratio of you antennas at any frequency or it it will oscillate! Seperating the antennas by distance or placing the RX antenna on the side of the hill facing the TX and the other antenna on the other side of the hill will allow you to use much more amplifier gain.If you have a passive system only with UHF band 5 antennas and the nearest phone cell in the direction of the TV TX it will extend the range of you mobile phones into the valley!
I might comment further that the 5 km is a fairly long
distance from the passive repeater.You can calculate the received signal strength based on the losses over the
5 km and the antenna gains you have to see how much input you need at the antenna to get a good signal. If you find you need more than 120 dbuV you might need to use the Hills "self help" TX units (up to 2 watts) which are a
bit more expensive but may be the cost could be shared by several neighbours.
I calculate the free space path loss at 600 MHz over 5 km is 102db. If you had a 20 db gain antenna at both ends, and you need a minimum of 50 dbuV into you masthead pre-amp at the RX location, then you will need 50 + 62 = 112 dbuV into the repeater "TX" antenna. If you are receiving 80 dbuV
from the repeaters "RX" antenna, then you will need at least 42 db of amplifier gain in your repeater. If the two antennas at you repeater have a front to back ratio of 25 db each and they are aimed close to 180 degrees from each other it will work but you will need a couple of wavelengths
between their backs. A 1 m x 1m piece of galvanised mesh (say 25 mm x 25 mm squares) mounted on the mast between the two antennas (back to back) might work also.
As you can see from the calculations, with less antenna gain and front to back ratio, it is more difficult.
The more signal you can get from the originating TX at your repeater site, the easier it will be.
If you have a field strength meter, go and measure how much signal you can RX at you proposed repeater site.If you down load the tv_8.pdf file from
and search through it to find the EIRP and frequencies of the TV stations you are trying to watch then you could calculate the likely signal strength at you repeater location if it is a clear line of sight and you know the
distance.For example, at 600 MHz (middle of the UHF TV band) if you were getting the signal from a 1000 W EIRP (Effective Isotropic Radiated Power = TX power x TX antenna gain) transmitter and your repeater location is 20km from this TX and you are using an antenna which has 20 db gain, then you will get 63 dbuV of signal which means your repeater needs at least 59 db of gain which means it would be a bit difficult to avoid feed back unless the RX antenna
was on the other side of the hill to the TX antenna. Hopefully you will have more signal than that.
If it is a 100000 W TX 20 km away, then you will RX 83 dbuV at you repeater site (20db RX antenna)and only 39 db anplifier gain is needed so you might be able to set that up on a single mast. How much input signal is the question - measuring it will give you the best idea of what is possible.It would be possible for the two UHF channels using Jaycar LT-3182 antennas but you just won't have anywhere near enough forward gain at VHF.Don't waste you money on the LT-3155 they won't work. They qoute 4db to 9 db of forward gain at VHF which means at 200 MHz you would have to have a 6 Watt transmitter to cover the 5 Km if you had 9 db of gain or a 60 Watt transmitter if there is only 4 db of gain at those frequencies! Remember, most mast head amps can only output 120dbuV maximum which is 0.2 Watt (200mW). Also, there is no where near enough gain privided by the mast head amplifier you propose.To work for the UHF channels only, you would need 2 x 20 db gain satellite line amplifiers such as a SAT1700 ($24.50ea)from WES Components. (I have tested these on a network analiser and they have more than 20 db of gain down to 400 MHz even though they are specified for 950 MHz to 2050 MHz - they work well for UHF TV) You also need a power supply and inserter such as a PSK18 ($43.55) from WES Components. (The prices are from there 1999 catalogue)I might not be worth it for just two UHF channels - you need to evauate that.On 569 MHz at 3278688 W EIRP a direct signal 140 km away with an antenna with 16db gain would give you 78dbuV. 42db of amp gain into an other 16 db gain antenna then 5 km to another 16db gain antenna would give you 50 dbuV - the minimum I would recomend to feed a mast head amp.
The JAYCAR LT3182 (91 element UHF) which is specified as 15-19db of gain should be able to do this. For the 2 UHF channels. Up-converting the VHF channels to UHF and combining them is one way of getting them across the 5 km.
so maybe it isnt as easy as i thought
any comments would be welcome
thanks ian
