Question about combining signal from 2 Yagi antennas into one coax

[JoshuaHufford]

I recently saw a friend setup 2 Yagi antennas aimed almost 180 degrees apart then hooked to a 2 way power divider with 2 equal lengths of coax. I had heard that this could be done but I'd never seen it in person before, he mentioned that the two coax cables that connect from the antennas to the power divider must be the exact same length which makes sense to avoid phase cancellation.

I would like to do the same for my setup but I have a few questions.

Is there any signal loss compared to a single yagi setup and if so how much? I would assume there would be some loss in the extra connectors.

Do they have to be aimed 180 degrees from each other or can you do other angles? For what I would want the angle would only be about 70 degrees.

If I can do about 70 degrees if they are on the same mast how much height separation between the two antennas would be needed?

Any other disadvantages to this setup?

 

[prcguy]

If you point two Yagi's away from each other and split the signal to them you will loose at least 3dB of signal. So if you have two 10dBd Yagi's you will end up with about 6.5 to 7dBd gain Yagi's. You won't need any height separation even at 70 degrees.

 

[JoshuaHufford]

Thanks, figured there had to be more two it.

I should have also mentioned this is a receive only setup.

 

[Ubbe]

Maybe it could be a good idea to connect an odd multiple of an electrical 1/4 wave of 75 ohm RG6 to each antenna and combine them. Then they would have 50 ohm to the main coax and a 1/2 wave between antennas that will isolate them between each other. That would probably give less loss than 3dB from a splitter connection.

The principal are explained here Stacking 6M Yagi (qsl.net)

/Ubbe

 

[prcguy]

The very best you can do with any kind of power divider is 3dB loss per antenna and it will probably be more.

Maybe it could be a good idea to connect an odd multiple of an electrical 1/4 wave of 75 ohm RG6 to each antenna and combine them. Then they would have 50 ohm to the main coax and a 1/2 wave between antennas that will isolate them between each other. That would probably give less loss than 3dB from a splitter connection.

The principal are explained here Stacking 6M Yagi (qsl.net)

/Ubbe

 

[Ubbe]

Power divider or isolating splitters are useless as they have more than a 3dB loss, the max gain you get by stacking two antennas. Stacking harness that are used when you combine several antennas do not have that kind of loss. When looking at professional antennas that are stacked it seems to be almost no loss at all.

/Ubbe

 

[JoshuaHufford]

Thanks guys, it wouldn't be a huge deal to just run another coax feed, I just thought if there was an easy way to combine the signals with little loss it would be worth a try. The signals are faint, hence the use of a yagi, I don't want to introduce any loss.

 

[prcguy]

Its impossible to have less than 3dB loss, even with a tuned 75 ohm phasing harness. You start with 100 watts, you split to 2 antennas, each antenna gets a maximum of 50 watts, that's all. If you start with 100 watts and both antennas get 53 watts each or 60 watts each, then there is an amplifier in the splitter because its impossible to have any less than 3dB loss for a 2 way splitter or divider or phasing harness or whatever you want to call it.

Power divider or isolating splitters are useless as they have more than a 3dB loss, the max gain you get by stacking two antennas. Stacking harness that are used when you combine several antennas do not have that kind of loss. When looking at professional antennas that are stacked it seems to be almost no loss at all.

/Ubbe

 

[Ubbe]

Its impossible to have less than 3dB loss

If you stack two antennas theoretically without any loss you doble the signal, 3dB, due to the directivity gets more narrow, usually flatter to ground and not up in the sky. If combining two antennas always have a 3dB loss, then it would be no point in stacking them.

That guy in the link had 5dBd gain from one loop antenna and combining that with another loop gave 7,3dB, a loss of 0,7dB in the combining process according to eznec and a total increase of gain of 2,3dB.

If having two yagi antennas pointing in different directions would give a 0,35dB loss from each antenna if that same stacking technique where used.

/Ubbe

 

[prcguy]

I think we are looking at the same loss but in a different way. The device used for splitting power to two antennas will always have a minimum of 3dB loss plus a few tenths of a dB more depending on design, etc. So for example, 100 watts into the divider or power splitter will give 50 watts maximum to each antenna but usually a little less.

Combining two identical Yagi antennas at the proper spacing will give you 6dB of additional antenna gain but you have lost 3dB of that in the power divider, so the net result is a maximum of 3dB gain for the power divider and stacked antenna gain together and usually a little less. Physics dictates the loss in the power divider and you cannot improve on the 3dB minimum loss. Depending on how you stack and phase the two Yagi antennas you can achieve the maximum theoretical gain of 6dB but its usually worse, especially by a non professional.

If you stack two antennas theoretically without any loss you doble the signal, 3dB, due to the directivity gets more narrow, usually flatter to ground and not up in the sky. If combining two antennas always have a 3dB loss, then it would be no point in stacking them.

That guy in the link had 5dBd gain from one loop antenna and combining that with another loop gave 7,3dB, a loss of 0,7dB in the combining process according to eznec and a total increase of gain of 2,3dB.

If having two yagi antennas pointing in different directions would give a 0,35dB loss from each antenna if that same stacking technique where used.

/Ubbe

 

[majoco]

I remember a friend in the UK making a "6 over 6 slot fed yagi" which is effectively two yagi's fed with a skeleton slot radiator which gave considerable gain and no lossy combiners. A quick google fond this...

https://vk3atl.org/technical/skeleton_slot_1.pdf

which looks very similar.

 

[prcguy]

A number of companies made similar antennas here in the 1950s and 60s but not any more. There is no magic gain to be had with that type of driven element. Maybe the BW is wider and now that I think about it a couple of the mfrs were TV antenna companies that made some models for amateur radio. If the idea had merit we would see them being made by everyone today.

I remember a friend in the UK making a "6 over 6 slot fed yagi" which is effectively two yagi's fed with a skeleton slot radiator which gave considerable gain and no lossy combiners. A quick google fond this...

https://vk3atl.org/technical/skeleton_slot_1.pdf

which looks very similar.

 

[Ubbe]

Combining two identical Yagi antennas at the proper spacing will give you 6dB of additional antenna gain

You are saying that adding one more antenna will increase the receive signal 4 times, and comparing that to making a yagi twice as long will only give 3dB? If one antenna give you 1uV and combining them without loss would increase the signal to 4uV?

When using stacking harness it will be a 1/4 wave to the combining point and using a 75 coax to a 50 ohm antenna it will give100 ohm at the combining point. The 50 ohm coax at the combining point then takes 66% of the signal and 33% goes to the other 100 ohm coax to the other antenna.

If you have a 1/4 stub that are open at the end it will reflect the signal back in 180 phase shift and cancel the signal, it works as a notch filter. If it is a short circuit at the other end it will shift the signal 360 and the reflected signal will be added to the original one and work as a bandpass filter. Using a 50 antenna will be closer to short circuit than open ended so just a minor loss.

/Ubbe

 

[prcguy]

I should mention we are discussing dB power gain or loss in antennas or power dividers and that is a log 10 function where voltage is log 20 as in the difference between 1uv and 4uv is 6dB in voltage not power. The difference between 1uv and 4uv in power is 12dB.

So yes I am saying that adding one more (Yagi) antenna will increase receive signal 4 times or 6dB in power but you will loose 3dB or 50% of that in the power divider combining the two antennas onto one feedline. Combing four Yagi antennas will increase receive signal by 12dB but you loose 6dB of that in a 4-way divider leaving you with a net gain of 6dB. How do you think you magically get 3dB gain combining two antennas when using a divider that has 3dB of loss? Because there is 3dB more gain from combining two antennas that few people talk about and I'm here today talking about it.

A stacking harness using 75 ohm coax will present about 100 ohms to the 50 ohm source and about 50 ohms to each antenna. It has the same 3dB loss as a Wilkinson or 3dB hybrid or any other type of 2-way power splitter, it will loose 3dB at each output. If you split or divide an RF signal to two different places and with equal division you loosed 3dB or 50% of the power at each output of the divider.

You are saying that adding one more antenna will increase the receive signal 4 times, and comparing that to making a yagi twice as long will only give 3dB? If one antenna give you 1uV and combining them without loss would increase the signal to 4uV?

When using stacking harness it will be a 1/4 wave to the combining point and using a 75 coax to a 50 ohm antenna it will give100 ohm at the combining point. The 50 ohm coax at the combining point then takes 66% of the signal and 33% goes to the other 100 ohm coax to the other antenna.

If you have a 1/4 stub that are open at the end it will reflect the signal back in 180 phase shift and cancel the signal, it works as a notch filter. If it is a short circuit at the other end it will shift the signal 360 and the reflected signal will be added to the original one and work as a bandpass filter. Using a 50 antenna will be closer to short circuit than open ended so just a minor loss.

/Ubbe