two antennas

[dark]

I have two Yagi antennas with same gain and operating at same frequency. Can we join these antennas to a transceiver , if yes what is required .. a splitter..?

Rgds

 

[davenn]

yes you can, you need a phasing harness, If the Yagi's are normally fed with 50 Ohm coax then you make a phasing harness out of two electrical 1/4 wavelength ( at the freq of use) lengths of 75 Ohm coax. to a central T connector then 50 Ohm coax down to the transceiver.

ok the 2 x 75 Ohm lengths of coax act as impedance transformers increasing the impedance from 50 to 100 Ohms. So when these 2 75 Ohms ends are tee'ed together
(2 x 100 Ohms in parallel) we again get 50 Ohms to match to the feedline to the transceiver

ok for getting lengths will vary a bit on the type of 75 Ohm cable and the freq you are using ( you havent said)

wavelength λ(metres) = 300/ freq in MHz

eg. λ = 300 / 144 MHz = 2.083metres

divide by 4 = λ/4 wave = 2.083 / 4 = 0.52m
now we need the velocity factor of the 75 Ohm coax you will need the specs on the cable you use. it might be 66% (.66)

so 0.52 x 0.66 = 0.344metres

there's a start for you

ohhh .... say you have a 8 element yagi that will be ~ 10 dBi gain
phasing a second 8 ele yagi will give an additional 2.5 - 3 dB of gain, is approximately doubling the gain.

The last thing to comment on is spacing of the 2 yagis.

As a rule of thumb, ~ 1.25 - 1.5 wavelengths is the norm. This give the best tradeoff between greatest forward gain and smallest sidelobes

cheers
Dave

 

[dark]

Thanks for the explanation, I am using 868MHz frequency and the impedance is 50 Ohm . Actually I was thinking to point my antennas in different directions (180 degrees back to back) , this is also a two way communication project TX and RX aswell . Is there any standard product available in market for impedance matching or splitting etc .
Rgds

 

[davenn]

ok, thats alright to point them in different directions. In that situation just divide your transmitter power by 2 ie. each antenna sees 1/2 the total power minus feedline loss and you just multiply that by the gain of the antenna.

just as an example 20W TX - say 3dB feedline loss (for ~15m of LDF4-50) so you end up with 10W at the T connector, so each antenna will see ~ 5W. Then 5W times the gain of the antenna say 10dBi (x10 gain) so you would have ~ 50W EIRP.

At 868MHz and anything over 10metre run of coax you wouldnt want to use anything less than LDF4-50 else feedline losses will be very high.


868MHz that must almost be in the cellphone band, if so there may be commercial phasing harnesses available.

cheers
Dave