----- Original Message ----- From: "Joop & Tineke Verdoes" [email protected] To: [email protected] Sent: Tuesday, November 21, 2006 1:33 PM Subject: [amsat-bb] Relative merits power divider vs power divider/combiner
OM-s As I got so much truely valuable and usable feedback on my hybrid power divider/combiner questions, I now dare to ask the following.
For making circulary polarised yagis one needs to split 'things' over two yagis
Hi Joop, PA1JAV
You need to split the TX power in two equal parts (3 dB) and to generate circular polarization the above equal parts of power must be radiated 90° out of phase by the two yagis. This can be accomplished by mounting the yagis in the same plane over the boom and feeding one yagi with a 90° delay line (1/4 electrical wavelenght long) or mounting the yagis over the same boom with 1/4 wavelenght physical offset and feeding the yagis in phase i.e. with the same lenght of feed lines for both yagis. If you do this the signal that can be received by the two yagis in RX conditions will be circularly polarized as in TX conditions and with the same sense of polarization.
There are basically two ways to split power A- use a 36 ohm power splitter, often a piece of waveguide with one connector on one end(for the TRX) and two connectors (for the Yagis) on the other end
The above A-power splitter has a characteristic impedance of 36 ohm it is a transmission line 1/4 electrical wavelenght long and during TX has the property to transform the 50 ohm impedance of the feed line in to the 25 ohm impedance of two 50 ohm yagis in parallel.
During RX conditions the 36 ohm matching line has the property to transform the 25 ohm impedance of two 50 ohm yagis in parallel in to 50 ohm impedance of the feed line and infact SQR (50 x 25)= 35.36 ohm or about 36 ohm
If the impedances as seen at the end of the above 36 ohm matching line are 50 ohm at one side and 25 ohm at the other side then in TX conditions the available power will be divided by two (3 dB) and 50% of the total power will feed each yagi while in RX conditions the power received by each yagi will be added in phase and applied to the 50 ohm line feeding the RX
By the way in TX condition or in RX condition the SWR of the 50 ohm lines feeding both yagis and the SWR of the main line feeding the TX /RX is 1
Since the 1/4 wavelenght long matching line operates the impedance transformation between 50 to 25 ohm and between 25 to 50 ohm then the SWR inside of it is 50/36 or 36/25 = 1.41 but only inside of it so that the losses of the matching line due to the SWR are negligible
B- use a 70 (75) ohm hybrid power splitter/divider, which has 4 ports, one extra for a terminator. The TRX sees just 50Ohm.
A- and B- are both fine for SPLITTING power. In both cases the TRX sees 50ohm, although there are two 50ohm connected in parallel and the other side of the divider. So both are fine for transmit.
A- and B- are also fine for receiving because the impedance transformations are working in both directions and the performance in TX and RX conditions is the same.
But...... A- Is ONLY a power splitter seen from the transmitter's side, it does present a mis-match as seen from the yagi, so on receive things are less than optimal
Disagree: A- is a 1/4 wavelengt 36 ohm matching line between 50 and 25 ohm. This line as seen from TX to the yagis is a power splitter and if the impedances of the two yagis are 50 ohm then the power flowing to the yagis is splitted in to two equal parts in phase and than radiated while no power is dissipated in the 50 ohm termination at 4
The same 36 ohm matching line as seen in RX conditions from the yagis is a combiner because the received power from each yagi is available at the 25 ohm point and is added in phase by impedance transformation in to the 50 ohm feed line going to RX and so the performance of the power splitter/combiner is the same in TX and in RX conditions.
B- Does work both ways, it presents a nice 50ohm port to the yagi, but the RX signal is split between the RX and the other yagi, so the signal that goes to the RX is 3db down.
Disagree: Here is a drawing of the hibrid ring power divider/combiner in which all 6 sections are 1/4 electrical wavelenght long lines made of 75 ohm coax cable 3 /°\ / \ / \ / \ 4 50 ohm 2 °| |°----////----| | | | | 1 °\ / \ / \ / /
Connect a TX in 2 with a 50 ohm yagi in 1 and the other yagi in 3 In TX conditions if the TX power is 100 watt the power appearing at 1 and 3 will be 50 watt in theory with the same phase and the power in 4 is zero.
Connect a RX in 2 with the same 50 ohm yagi in 1 and the other yagi in 3 If the lenght of the lines feeding both yagis are the same lenght and if they are connected to the same hot side of the driving elements then the received signals available from both yagis are applied to 1 and 3 with the same phase and their sum appear in 2 in phase without any 3 dB loss while the power at 4 is zero (or 0 volt)
The SWR = 1 in to all outside 50 ohm lines connecting the yagis to the hybrid and the hybrid to TX/RX while the SWR inside the ring of the hybrid is 75/50= 1.5
Does anybody have a view on how "bad" A- is vs B-?
Method A can be better than method B or vice versa depending on the specific use we need
Have fun Joop, PA1JAV
We already discussed this matter on BB the 26 may 2006 with messages titled " Power Dividers fo X-Yagis" wich are available from the AMSAT-BB archives but at that time the theme was centered on the comparison between hibrid-rings and Wilkinson divider/combiners.
Best 73" de
i8CVS Domenico