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Author Topic: How to feed 2m/70cm dual yagi?  (Read 2576 times)
AA3YV
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« on: August 06, 2005, 11:36:14 AM »

Hello.  I'm new to sat operation and I just built a 70cm amsat version of the Cheap Yagi (from plans at http://www.clarc.org/Articles/uhf.htm).  It performs great! Now I want to add a 2m 3-elemet yagi to the same boom.  Can somebody explain how can I feed both of these antennas from a single HT?  I want to transmit at 2m and recieve at 70cm and I do NOT have a full duplex HT.  Is there any way to feed both of these antennas using a single coax without using a diplexer?  73. Vasiliy
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K5LXP
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« Reply #1 on: August 06, 2005, 03:35:51 PM »

It will not be as easy to work the sats using a half duplex HT, but since your radio is half duplex,
you could use a coaxial relay. But a good one would cost you more than a diplexer. There aren't too many
ways around it, just buy a diplexer. Arrow makes one for their 2M/440 yagi or you could go with one
meant for mobiles if you ever anticipate running high power.

Mark K5LXP
Albuquerque, NM
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ZS1AN
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« Reply #2 on: September 16, 2005, 03:18:04 PM »

Hi Vasily

This may work, depending on the impedances of the two yagis. I originally wrote it as a response to a similar question in another discussion forum.

Put the two yagis, one for 2m and one for 70cm, on a single boom, but cross polarised to minimize interaction (i.e. on is horizontally polarized, the other is vertically polarized). Both yagis are assumed to have 50 ohm impedance at the design frequency.

Now measure or calculate the input impedance of the 2m yagi at 435 MHz (middle of 70cm band). Hopefully this will be something other than 50 ohms - ideally we would like a SWR of 10:1 or higher, although 5:1 would probably suffice. Use a Smith Chart to calculate how much 50-ohm coax is required to transform this impedance to resonance (i.e. no reactive component) with a high resistive component, and attach this much coax to the feedpoint of the 2m beam. Measuring the impedance at the other end of the coax, you should find it has a high resisitive component (500 ohms if the original SWR was 10:1).

Repeat the process, calculating or measuring the input impedance of the 70cm yagi on 145 MHz and using the Smith Chart to calculate how much 50-ohm coax is required to bring it to a high resistance, zero reactance point. Attach this much coax to the feedpoint of the 70cm yagi (taking into account the velocity factor of the coax, of course).

Then simply connect the free ends of the coax cables in parallel, and feed the combined antenna at this point. (You could use a "T" connector if you wanted). Each will have a 50 ohm impedance on the band for which the yagi was designed, but a high impedance (500 ohms or more, with luck) on the other band. This means that 90% of the power will go to the "correct" yagi, with 10% going to the other. Admittedly the 10% that goes to the "wrong" yagi will probably spoil the f/b ratio, but it will have very little effect on the gain of the yagi - less that 0.5 dB down if the impedances on the "wrong" bands are 500 ohms or more.  

Note that if the SWR of either antenna when measured on the "wrong" band is less than 5:1 then this method won't work, you'll need a proper duplexer.
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