Yagi Material and Dipole Questions

[matters]

Hi guys,

I've been listening to scanners for a long time now. I've really only used the stock or telescopic antennas, so I don't know a whole lot about antennas. I'm planning on building a Yagi antenna for 772 MHz to monitor my local P25 system. The tower is 10 miles from my house. I've seen multiple designs online using different materials and dipoles.

My first question is what material should I use for the boom? I've seen PVC, dowel rods, 1x1 pieces of wood, and aluminium square tubes. From my understanding, only the dipole/driven element needs to be isolated/insulated from the other elements (which do not need to be isolated, but can be). Is that correct? Do the materials have pros and cons relative to the signal or is it just preference?

I have also seen different types of dipoles for the driven element. Which would be best to use? I'm don't know the proper names, but I have seen straight dipoles that stick out like the other elements, folded dipoles that loop around almost all the way, and one that sticks out on one end and loops around on the other. I'm thinking of using 1/4 inch aluminium rods, but how hard is it to bend? Is it best to coat everything with paint or leave it bare metal?

Sorry if this is off topic, but what how far off frequency could I go with a Yagi? I've read that they were only good to up to around 1% off the tuned frequency, but antennas sold online are advertised with a larger frequency range/bandwidth. Either way it is not going to be an issue, just something that I was wondering about.



I like what was done in this thread:
https://forums.radioreference.com/build-your-own-antenna/125567-homebrew-850mhz-yagi.html

I'm going to use one of these calculators:
Online Calculator .:. Yagi Uda Antenna
or
Martin E. Meserve - K7MEM - VHF/UHF Yagi Antenna Quick Designer



What your guy's input? Let me know if you need to me clarify anything.

Thanks.

 

[alabamarailfan]

Hey there,
In no way to I want to stifle your creativity or enthusiasm as building antennas is a great hobby.

However, I wanted to show you these. I bought one and it works great picking up a 700 MHz P25 install about 12 miles from here. I get 5 full bars and a very low error rate on my BCD996T. Just FYI! Check the price out! I was shocked!

Look at this on eBay YA7009F DMS Wireless 700MHz Directional Yagi Antenna with 9.0dBi gain | eBay

 

[mmckenna]

If this is going to be mounted outside, avoid wood. It'll absorb moisture, crack, etc. It'll work short term, but it's not ideal for long term.

PVC can be a good option, just be aware that some PVC pipe can break down with ultraviolet light.

Aluminum is good, but it can corrode over time. Not an issue if you protect the connections.

Stainless steel is ideal, but can be difficult to work with if you don't have the right tools.

Copper can be an option. Either heavy gauge solid wire or copper tubing/pipe. Nice thing about it is that you can solder it. Once soldered, it can corrode, but the soldered joints will stay good.

For the hobbyist, copper wire or aluminum and PVC might be your best choice. UV resistant PVC pipe would be a good idea. The grey PVC electrical conduit can be a good option.

 

[RF-Guy-El-Loco]

Yagi Antenna Builds

When you build the first one, you'll want to build 2 more. The PVC pipe or dowel rod idea works out okay, but I have found that your drill holes will almost always be slightly off (one side vs the other) on plastic pipe, no matter how good your markings are. Unless you can use a drill press. That will keep the holes in exact side to side alignment. I have scrapped many, but the material is so cheap and you can reuse most all of it. I also used cheap dial calipers with a digital readout for the mm measurements. You'll never get it exact of course, just pretty close. Then you eyeball it and you see it wasn't good enough.

Wood is good for the boom, the 1x.5 or .5x.5 inches is what I have used. And the holes came out straighter on wood. I have used coat hanger wire, #12 AWG solid wire and 1/4 copper tubing for the elements. The thicker tubing increases the bandwith, but don't fret that. The #12 solid wire bends/cuts easy. The copper tubing works good, looks good, I used a tubing cutter. Fluke281 (poster) here said to cut the tubing slightly too short and then insert sheet metal screws into the ends to adjust length. Great idea. You need to tweak these (never do it perfect) antennas.

The Driven element is split up, allow for the gap, but it is difficult to get the length right and affix the split rods into your boom permanently and in also in perfect alignment with other rods. You have 2 pieces you have to glue in and they don't like to Stay in position while the glue dries. The other reflector/directors are all one piece, no trouble there. The folded dipole driven element type probably works better as an antenna, but the radius bends are tricky, you will need to bend around a smaller dowel rod and it will take a few tries to get (the curves vs. the total length) right. I have one 10 rod yagi with the driven rod straight on one end and rounded other end, (folded dipole style) works fine on a .5x.5 inch wood boom. Also, soldering the balun or coax onto the wire rod will suck the heat right out of your iron, figure on 3 mins or more/side or use a 100 watt big soldering gun. The coax/balun will start to melt away on you with all that high heat near it too. And coax copper braid is basically just solder wick.

Painting it, your preference. If you cut for 772 Mhz you will still have good reception at 855 Mhz, but don't ask for 155 Mhz from it, (weak) because it will be so directional then and off cut freq. The other issue is design, the more directors, the more forward gain, 10 miles, anything will work, even just 3 total elements. I found a few different antenna designs in regards to rod-to-rod director spacing confused me, I tried each, I think the Yagi/Uda online calc will make you a fine antenna. And your 2nd, 3rd, as well. I have also built two (7 foot 855 Mhz cut) collinear antennas, they rx Omnidirectional great, (even 155 Mhz Wow, but they are pretty complex long things to build, a lot of hours).

Final things. Once you find the sweet position to use, be sure that your P-25 system is not using multiple sites/towers for just that One system, and the towers are not scattered around the map (of course they are). If you are pointed directly at one tower (10 miles out) with a super high gain 12 element directional yagi antenna and the above is true, you will want a yagi with less elements rather than more. But any antenna cut for the proper frequency will work even if it just pointed in the general direction of. Be sure to run at least good RG-6 coax to it for 50ft or less, as at 772 Mhz, you'll only loose a dB or two with RG-6 cable. Cheap coax signal loss increases dramatically with frequency. Don't use RG-58/59 coax, you'll loose half your signal. Also mount it with the rods vertical, mine work better 20 degrees off pure vertical. I also notice that the coax itself tends to act as your actual antenna at this frequency. Build a few different antennas, test out. You'll get a working knowledge of RF antenna designs and theory.

 

[matters]

Thanks for your inputs.

I looked at buying some online from eBay, but I decided to make one myself.

I have spare PVC that I will use for the boom and build a makeshift stand with wood and more PVC for the time being. I went to the hardware store yesterday and picked up some 1/4" aluminum rods that I'll use for the elements.

I'm pretty sure this wont be my last antenna too. Depending on how well this goes, I might make a few more for different frequencies or change the design and compare them. I will have access to a drill press, so making the holes in the PVC shouldn't be too much of a challenge. According to the calculator that I linked said that the boom should be at least 2.2 wavelengths long, which puts it at about 3 feet with 10 elements. This seems overkill because the signal is fairly strong. The nice thing that I noticed is that the spacings and lengths remain the same if I want to add or remove them. That is, if I originally had 10 elements and wanted to make it smaller to 5 elements, all I would have to do is cut off the ones I don't want. The spacings for the first 5 elements don't change.

Do you think for my first build, a straight dipole would work fine?

 

[prcguy]

Metal vs insulated booms will cause the elements to be different lengths, so make sure your Yagi calculator has an option for metal and will compensate for diameter. Element diameter will also have an effect on the element lengths, so hopefully you can calculate everything for the materials you use. At 700MHz a very small, like 1/4" or less change in something will have a noticeable effect on performance.
prcguy

 

[lmrtek]

for $10.99 you can buy a 2 bay bowtie TV antenna that will perform quite well and is fed with low loss and inexpensive rg6

it covers 460-800mhz

or you can easily make a multi bay bowtie with coat hangars and a stick

There are plenty of you tube videos on their construction and performance

 

[zz0468]

For consistent results, high performance, and ease of construction, I prefer a corner reflector for 700-800 MHz home built antennas.

Yagis can be rather complex, considering that materials and diameters can have a significant impact, and everything affects everything else. Unless you duplicate someone else's design almost precisely, your measured results will be different. If you use an online calculator, build it precisely as designed.