Combining two vhf antennas, one yagi and one omni?

[Rawkee1]

Thanks so much PRCGUY, thats awesome. It looks like a great educational project to take on. I remember those 278- catalog numbers from Radio Shack. I was a RS manager many moons ago! I wish Radio Shack was still around,

 

[majoco]

I would like the see the phasing cableform for all those antennas!
Trainsofthought said:

I've seen these exact dipole arrays on commercial/municipal towers in the area...pointed 180 degrees apart

Our base was in the middle of two air routes, one north and the other south and probably 150km each, say 90miles. Our boss came up with a 300ohm folded loop about 2feet each leg mounted on a long boom with the loop in the middle and a couple of directors on both ends. Great in theory - but did it work? Yes, and very well although we had to find a bit of fibreglass pipe for a mast extension as the existing metal mast made like a reflector which favoured the north too much. No need for phasing, just a 300 to 75ohm balun. We could talk to our aircraft almost as soon as they got to about 5000ft heading our way from both directions.

 

[Rawkee1]

What yype yagi would you recommend? It's cheap enough to try. Those folded dipoles I can buy from Ukraine, but pricey. $150 each so $600 for a for dipole array.

 

[G7RUX]

Not to mention a 3dB decrease in received signal level due to the splitter and the two antennas are pointing opposite directions.

Indeed, although 3 dB isn’t a huge problem unless you are trying to work very weak signals.
(The splitter would be used in reverse so, if a hybrid rather than a simple resistive unit, should provide a little isolation by itself.)

 

[G7RUX]

What is the trick, if possible, to setting up a yagi and an omni for VHF155-161Mhz combined to come down one line to one radio?

Another simpler way to give a bit of directionality but without cost and complication would be to use an antenna like a simple 3 element yagi but with equal-length elements; this would give lobes in each direction with reasonable directionality and without the need for combiners, etc. The gain in each direction wouldn't be very high but would work pretty well I would think.

 

[nickwilson159]

If DPD Productions still made these, I would’ve said go this route:

DPD Productions - TrainTenna Railroad Band Antennas for Commercial & Hobbyist Applications

 

[Ubbe]

Mobile/Base scanners should have several antenna ports that where diode switched and each channel frequency have a setting to tell which antenna to use. It would help in those many cases when you need to use a specific antenna for a system or frequency band. It wouldn't cost much to add that in the initial design phase.

You could do a poor mans solution by using the attenuater signal and cut the copper trace to the actual attenuater circuit and instead use that signal to a diode switch to select between two antenna ports, or use the signal to an external box with a diode switch. But usually the whole front end circuit in a scanner are covered in epoxy and makes it more difficult to find the correct trace.

/Ubbe

 

[Rawkee1]

Mobile/Base scanners should have several antenna ports that where diode switched and each channel frequency have a setting to tell which antenna to use. It would help in those many cases when you need to use a specific antenna for a system or frequency band. It wouldn't cost much to add that in the initial design phase.

You could do a poor mans solution by using the attenuater signal and cut the copper trace to the actual attenuater circuit and instead use that signal to a diode switch to select between two antenna ports, or use the signal to an external box with a diode switch. But usually the whole front end circuit in a scanner are covered in epoxy and makes it more difficult to find the correct trace.

/Ubbe

I totally agree with you. Bearcat put out a survey, before the SDS scanners made it to the market, and asked people for suggestions for the future. I wrote to them and suggested multi antenna ports along with making scanners in the future, be able to use a remote like a TV for a variety of functions like volume, mute, display color & brightness ect. I guess my suggestion made it to the circular file.

 

[G7RUX]

Mobile/Base scanners should have several antenna ports that where diode switched and each channel frequency have a setting to tell which antenna to use. It would help in those many cases when you need to use a specific antenna for a system or frequency band. It wouldn't cost much to add that in the initial design phase.

You could do a poor mans solution by using the attenuater signal and cut the copper trace to the actual attenuater circuit and instead use that signal to a diode switch to select between two antenna ports, or use the signal to an external box with a diode switch. But usually the whole front end circuit in a scanner are covered in epoxy and makes it more difficult to find the correct trace.

/Ubbe

If one wanted to go the switched route then it might well be more accessible to use an external switch since, for receive at least, they are cheap and simple and don’t require performing surgery on your radio.

Since the OP had indicated that this particular setup was only used for listening to this railroad system from opposite directions then I wouldn’t expect that anything quite so drastic should be necessary; a simple antenna should do the job pretty well, even combining two shouldn’t be too bad at all.

I have used combined antennas before and provided you are careful in the setup it works well…I used a pair of antennas with decent preamps, combined at the receiver in a switching unit so either could be used separately if wanted. With a bit of jiggery-pokery it was possible to adjust the phase of each signal before the combiner using a pair of adjustable phase delays made using op-amps so cancellation of some interferers could be achieved.

To the OP: give it a go and see what happens since nothing much would be lost really.

 

[Ubbe]

If one wanted to go the switched route then it might well be more accessible to use an external switch since, for receive at least, they are cheap and simple and don’t require performing surgery on your radio.

But you are not going to be fast enough with your hands to switch over antennas while it's scanning different channels in different frequency bands and systems. Using a channel or department/group dependent flag will do the switching for you at scan speed. Most people do not use the attenuator, and that flag can then be used to control a switch. If you have a WFM setting for FM broadcast then that signal can instead be used but requires a lot more work to then force the scanner to NFM or FM mode while WFM are selected. It would be so much easier if the scanner where built with antenna switching in mind.

With a bit of jiggery-pokery it was possible to adjust the phase of each signal before the combiner using a pair of adjustable phase delays...

I used to do that by having to different antennas and then combined them at the scanner using a T connector. Then I had 3 different short lengths of coax, something like 1/8, 1/4 and 1/2 wave, that I used to add to one coax to make the signal in phase with both antennas to improve the signal strength.

/Ubbe

 

[G7RUX]

But you are not going to be fast enough with your hands to switch over antennas while it's scanning different channels in different frequency bands and systems.

The OP specifically said they were listening to one system and one alone.

 

[Ubbe]

Here is a random mfr but the same idea. A group of dipoles facing one direction and another facing 180deg opposite. It gives a figure 8 pattern with about 6dBd gain in each direction with four dipoles on each side. This would be 20 to 21ft tall at VHF and very broad band.

View attachment 135951

If you stack two dipoles you get a 3dBd gain. 4 dipoles gets you 6dBd.
If you stack all 8 of those dipoles facing the same direction you get 9dBd gain. You will have an almost full coverage east-west if you point them at north or south. It will be super expensive and if instead doubling the height of the mast will be cheaper and probably provide higher signal levels with a less costly antenna.
A00003_01.pdf (webb.co.za)

Using a $100 Diamond F23 that are 3x5/8 will get you a good gain and weigh almost nothing and can be put on top of that 20ft mast pole without any extra support needed as its wind resistance are very low.

/Ubbe

 

[Nm1hart]

What is the trick, if possible, to setting up a yagi and an omni for VHF155-161Mhz combined to come down one line to one radio?

On our repeater we are using and Omni antenna teed off to a directional to get over the mountain in one direction. We literally used the same set up truckers use to split there antennas. It does work good swr has to be turned in. Last issue of qsl magazine had a tunner you could make your self to accomplish this.