DIY Yagi Antenna Question

121940kz · Apr 11, 2015

I'm getting ready to build my own 4 element yagi antenna, using the calculator on this website. 4 Element Yagi Antenna Calculator: i1wqrlinkradio.com

I'm using a 5/8" dowel, and 3/32" brass rod from hobby lobby. The main reason i am building this is because the county i like to listen in uses P25 simulcast, and my scanner drops everything like crazy. It is horrible how bad it drops literally every conversation. I hear a few words and then it drops. I am also using the stock antenna on my PSR-500 so i know that doesn't help either.

My 1st question is, would adding more director elements be beneficial? Or are the 2 directors good enough. If i wanted to add more directors, would i just use the same length that the 4 element yagi calculator gives me, and then space them out according to the calculator? Or does adding more elements mess with the rest of the antenna calculations?

My 2nd question is, i have a plethora of RG-59 75ohm cable, which i was planning on using for this project. Since this is only a receive antenna, and the cable length is probably only 6 feet, does it make a big difference that i am using 75 ohm cable instead of 50 ohm?

Thanks for anyone's input!!

ab5r · Apr 11, 2015

Please, give us an idea of the DISTANCE that you are trying to listen too, and what kind of terrain (flat, hilly, mountains etc.,).

Generally speaking, RG-59 is not that great of loss for receiving, depending upon length of run.

121940kz · Apr 11, 2015

ab5r said:
Please, give us an idea of the DISTANCE that you are trying to listen too, and what kind of terrain (flat, hilly, mountains etc.,).

Generally speaking, RG-59 is not that great of loss for receiving, depending upon length of run.

The nearest tower is about 5 miles i believe. And fairly flat terrain (Kansas).

ab5r · Apr 11, 2015

IMHO, what you are planning is fine. Just get it as high as you can. That matters too.

121940kz · Apr 11, 2015

ab5r said:
IMHO, what you are planning is fine. Just get it as high as you can. That matters too.

What about the number of directors question I had? Do you have any insight on that? Thanks!

ab5r · Apr 11, 2015

It may decrease the overall Bandwidth, narrowing it to a more specific frequency, but not help with sensitivity. I don't see any advantage in doing that.

A Yagi generally is designed for a particular frequency, but receives well on others (bandwidth). For your use, the four should be fine. BUT, you can always experiment. Good Luck

popnokick · Apr 11, 2015

5 miles? Signal strength is not very likely to be the problem you are experiencing. More likely: multi path simulcast from several towers on the system transmitting the same signal simultaneously. Documented extensively by others here on RR. The Yagi may help your receiver "focus" on a single tower, but if there is another tower inline in the same direction as the one that is nearest to you, the problem may get worse. Search "simulcast" and "LSM" here on RR to find the many different approaches being taken to resolve this problem.

121940kz · Apr 11, 2015

popnokick said:
5 miles? Signal strength is not very likely to be the problem you are experiencing. More likely: multi path simulcast from several towers on the system transmitting the same signal simultaneously. Documented extensively by others here on RR. The Yagi may help your receiver "focus" on a single tower, but if there is another tower inline in the same direction as the one that is nearest to you, the problem may get worse. Search "simulcast" and "LSM" here on RR to find the many different approaches being taken to resolve this problem.

I know the signal strength is not the issue it's the multi path distortion. I'm wanting to use the yagi In hopes to block out the other towers interfering.

slicerwizard · Apr 12, 2015

ab5r said:
IMHO, what you are planning is fine. Just get it as high as you can. That matters too.

I don't think you understand the OP's problem.

To the OP, you probably want more elements, but you can always experiment. Be aware that how you connect your coax to this antenna can greatly affect its directionality - do it wrong and you'll still have near-omnidirectional performance.

121940kz · Apr 12, 2015

slicerwizard said:
I don't think you understand the OP's problem.

To the OP, you probably want more elements, but you can always experiment. Be aware that how you connect your coax to this antenna can greatly affect its directionality - do it wrong and you'll still have near-omnidirectional performance.

Center lead goes to one half of the driven element, and the coax shield goes to the other half. The 2 elements are not touching. Is this what your talking about?

121940kz · Apr 12, 2015

slicerwizard said:
I don't think you understand the OP's problem.

To the OP, you probably want more elements, but you can always experiment. Be aware that how you connect your coax to this antenna can greatly affect its directionality - do it wrong and you'll still have near-omnidirectional performance.

prcguy · Apr 13, 2015

Something to consider is the driven element starts where the coax splits to each element. I see the braid and center conductor look about an inch long in the picture before connecting to the elements. If the formula calls out for say 3.00 inch driven elements, the long coax leads are making the elements more like 4.00 inch long each and the driven element could be resonating 100MHz or more lower than you want.

At HF through VHF, lead lengths are not usually a problem but at UHF and above its a big problem.
prcguy

121940kz said:

wa8pyr · Apr 13, 2015

ab5r said:
IMHO, what you are planning is fine. Just get it as high as you can. That matters too.

Since he's trying to limit reception to a single site due to multi-path distortion, getting it up higher would actually hurt what he's trying to accomplish (limit reception to a single site and eliminate all others as much as possible).

121940kz · Apr 13, 2015

prcguy said:
Something to consider is the driven element starts where the coax splits to each element. I see the braid and center conductor look about an inch long in the picture before connecting to the elements. If the formula calls out for say 3.00 inch driven elements, the long coax leads are making the elements more like 4.00 inch long each and the driven element could be resonating 100MHz or more lower than you want.

At HF through VHF, lead lengths are not usually a problem but at UHF and above its a big problem.
prcguy

That's weird that no sites I have been on for measurement calculations mention that, but it makes sense. Also, does it matter which end is sticking up vertically? As in the center lead on top versus the shielding on top?

If I wanted to add more directors could I make them the same length as the other directors and keep the spacing the same, and just simply add more to narrow the radiation pattern? or would I need to recalculate other elements due to adding more. Thanks for your knowledge.

slicerwizard · Apr 13, 2015

121940kz said:
Center lead goes to one half of the driven element, and the coax shield goes to the other half. The 2 elements are not touching. Is this what your talking about?

As noted by pcrguy, you may have a problem. Try connecting that antenna to a receiver that has a decent RSSI display and verify that the antenna is actually directional. A DVB-T dongle would be perfect for the job.

bharvey2 · Apr 13, 2015

I've been experiencing the same problem that you have with simulcast distortion. I've tried a commercial yagi antenna but found that a homebrewed corner reflector provides a better signal, at least in my application. I also used 75ohm coax and found that using a 300ohm to 75ohm balun/transformer made a substantial improvement over directly connected coax. I suspect that not using the balun causes the shield to act as part of the antenna. FWIW, a 45 degree corner worked better for me than a 90 degree corner. Oh, and changing antenna locations can make a big difference too.

bharvey2

121940kz · Apr 13, 2015

bharvey2 said:
I've been experiencing the same problem that you have with simulcast distortion. I've tried a commercial yagi antenna but found that a homebrewed corner reflector provides a better signal, at least in my application. I also used 75ohm coax and found that using a 300ohm to 75ohm balun/transformer made a substantial improvement over directly connected coax. I suspect that not using the balun causes the shield to act as part of the antenna. FWIW, a 45 degree corner worked better for me than a 90 degree corner. Oh, and changing antenna locations can make a big difference too.

bharvey2

What did you make your corner reflector out of? That's always another option I'll have to explore if this antenna doesn't do the trick. Also did you use the corner reflector with the yagi you bought?

bharvey2 · Apr 13, 2015

The reflectors are made of two pieces of 1/6" aluminum sheet, each about 12" x 12". I ripped a length of 2"x 4" on a table saw to get a piece of wood with the proper angle on which to mount the aluminum sheets. I used some 1/16" stainless rod for the 1/2 wave dipole but heavy copper wire would work as well. Trial and error got me the distance between the dipole and the corner. Also, it's important that the connection to the dipole be straight out of the back as I understand that anything in front of the dipole can cause problems.

bharvey2

bharvey2 · Apr 13, 2015

Whoops, missed part of the question. I used either the yagi or the corner reflector. Never tried both together.

bharvey2

WB0VNE · Apr 18, 2015

I'm not an expert but don't you need a matching network? I built a bunch of 3 element Yagis where the dipole (two rods) were connected to the coax (*Just as you did) but there is a short piece of wire shaped like a the letter U. This is a Hairpin match. I can't tell for sure but is the element you wired your coax to a single piece of rod? I made some antennas using measuring tape from the concept here-> http://theleggios.net/wb2hol/projects/rdf/tape_bm.htm The YagiCad4.1 software works. I've used it to make antennas for 120 and 245 Mhz with different numbers of elements and they both worked really well.

Its quite fun to build something like this and have it work so keep at it and good luck!

DIY Yagi Antenna Question

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