I just ordered a 900Mhz yagi & have a few Q's

[N2UJZ]

I usually have a bit of trouble decoding the main site on the local P25 system (876MHz.) I just ordered a yagi (7 element) to hopefully increase reception, however I don't want to attenuate the other sites too much. The tower I'm having trouble getting is about 15 miles due E of me. There is another site about 4 miles to the south with not as much activity but I'd like to receive both without going up on the roof to mess with the antenna.

The antennas specs say it has 10dB gain & says "Horizontal plane 72°F ± 5°F" Is this the beams angle it receives best?

Here's a link to the antenna: Hot sale Outdoor directional Yagi antenna for cell phone boosters 800/850/900MHz | eBay

And a map showing my location (A) & the towers https://maps.google.com/maps?f=d&sou...&mra=ls&num=10

If pointed East, will the location to the south be severely attenuated even though its much closer?
Can I compromise by pointing the antenna between them or would that just defeat the purpose of a yagi?

Thanks for any help. -Justin N2UJZ

[benbenrf]

N2UJZ

Yup - it's going to be attenuated. The 2 beamwidth figures you have been given are for -3dB (thats the industry standard). In other words, signals the antenna picks up that are outside the given vertical and/or horizontal beam widths are going to be down in strength by at least 3dB, or are going to be at least 50% weaker. The problem is from East to South is 90degrees, so the best way to cover both these towers is to point the Yagi at 45degrees East of South, or 45degrees South of East. This will mean both towers are going to be no more than around 15degree - 20degree outside the 3dB beam width. If you point the antenna South, or East, the other tower will then be at least 50degrees outside of the beam-width, and that will result in a huge drop in gain with respect to the tower not been pointed towards.

To see exactly how much Gain is been lost you really need to see a copy of the radiation diagram. In any event the more outside the 3dB beam-width the more (and quicker) Gain tends to drop off.

Your ideal and best solution here is really 2 of these antennas connected together with an antenna combiner - a used one off eBay that covers these frequencies can easily be found for around $20 - $30, but failing this just point the antenna exactly 45degrees off South or East, between the 2. The front/back ratio is quite acceptable so the antenna is going to pretty much ignore everything else and you should enjoy a good quiet signal.

Overall it's not going to be the Gain you get much benefit from here, but the fact that you going to enjoy a pretty good signal to noise ratio - on condition there isn't a pesky radio transmission inside the antenna 3dB beam width that is coming from something you are not interested in listening to. If that happens, I'm afraid the towers will probably be well and truely shouted down.

[N2UJZ]

So since the far tower is 3X the distance compared to the one to the South (which has a very strong signal) it would be 9dB weaker, correct? So basically I just might be able to get away with trading some attenuation on the near tower for some gain on the far one- I guess I'll just have to experiment. I wish my PRO-2096 had an S-meter but the only way I can really gague signal strength is by ear & the % of decode

Most of the action here is on the far tower which is downtown, so there are other transmitters & towers nearby but I think they're far enough not to cause overloading

My current antenna is small vretical made by simply stripping the shield off of a few inches of coax & foil molded into ground plane elements based on this design Ground Plane photo - Dick Harris photos at pbase.com

It works surprisingly well for what it is but I want to get a better decode from the downtown site. Of course it always seems to get glitchy when something good is happening! (luckily for me local PD uses unencrypted P25 even on the Special Ops & investigations stuff, I want to enjoy it while I still can! -Justin

[WayneH]

I'm going to split these posts off to their own thread. Please create your own thread when you have your own question. Thanks.

[benbenrf]

I'll answer just the first part regards tower distance and attenuation: Yes, the further towers' signal will be weaker, but so long as it is within the receive sensitivity of your receiver then it's really a non-issue regards weather the signal is x dB or y dB in strength.

When it will make a difference is when the difference is such that the weaker signal is presented to the receiver with poorer signal to noise ratio (or carrier to noise ratio as it more accurately described with respect to P25 signals) i.e. boarder line cases that result in inconsistant demodulation. In those cases orientation of the antenna favoring the weaker signal over the stronger one could realise a hearable difference in the 2 signals. And that's exactly why using 2 Yagi's is going to be far preferable than using one.

The problem with S-meters is , as you have touched on, they tell you signal strength, but they do not tell you anything about signal quality: the signal may well be full of noise. That of course is no good. Yes - your ears and the % of decode is a better way of going about it.

[jackj]

All directional antennas have a major and several minor lobes. Without having a copy of the antenna's radiation pattern you can't tell of you can take advantage of this fact. You could possibly position the antenna so that the major lobe is pointing either at or just off to one side of the weakest station and a minor lobe toward the stronger one. Reputable manufacturers will have the antenna's radiation pattern on their web site or available my surface mail. See if you can get this info for your antenna.

[N2UJZ]

Well I got my antenna & all of the elements are in contact with the metal boom & the driven element isn't isolated. Also with a multimeter across the coax it reads 5 ohms. SoI think what I actually got is either designed by an idiot or just made to look like a yagi.

I should have looked at the picture or just maybe made it myself for less than the $20 I spent. Should I even bother getting the connectors & hooking it up? Or will it be utterly useless as I suspect?



Picture of it is here: Hot Sale Outdoor Directional Yagi Antenna for Cell Phone Boosters 800 850 900MHz | eBay

[Soundman]

If it look like crap I would say something .

[talkpair]

I think you'll find most commercially made yagi's have DC continuity between the driven element and the boom.

I would at least give it a try.

[benbenrf]

[QUOTE=N2UJZ;1868110]Well I got my antenna & all of the elements are in contact with the metal boom & the driven element isn't isolated. Also with a multimeter across the coax it reads 5 ohms. SoI think what I actually got is either designed by an idiot or just made to look like a yagi.

I should have looked at the picture or just maybe made it myself for less than the $20 I spent. Should I even bother getting the connectors & hooking it up? Or will it be utterly useless as I suspect?

N2UJZ

Don't worry - it's made correctly: as talkpair said the issue of continuity between the any/all elements and boom is a non-issue.

Set it up and let us know how things work out please.

[kruser]

Originally Posted by N2UJZ

Well I got my antenna & all of the elements are in contact with the metal boom & the driven element isn't isolated. Also with a multimeter across the coax it reads 5 ohms. SoI think what I actually got is either designed by an idiot or just made to look like a yagi.

As has been stated, many yagi's have shorted radiating or driven elements. It's pretty common practice especially when you start dealing with UHF and up. They may show a short with a meter but not to RF.

I have several yagi's from many different manufacturers and most are of the shorted design as can often be seen visually or when checking with a meter. I'd guess a DC shorted design also keeps static buildup to zero.

The one that works the best for me is an Maxrad 12 element that is shorted.
Another one that works very well also is a Maxrad 12 element (older model) but it has a little gamma match tuning stub that is not shorted and shows open with a meter.

I'd say it will work fine for you.

The problem I do have is most of my Yagi's only work well for the range of frequencies they were designed for (850 MHz center in my case). They work for the 700 and 900 bands but they seem to lose their directional properties pretty quick when I get outside the design range. Then they act more like an omni design.

That antenna in your link is rated in dBi so you will probablly see maybe 5 dB of gain.

[wingmaker]

Id bet you pointing at the 15 mile tower it will help, and you more than likely will still pick up the site that is 4 miles away. LOVE AND BLESSINGS

[N2UJZ]

Ok, thanks guys. I've only built antennas for HF & haven't ever come across a yagi like this one. I didn't think someone would go through the trouble to make a "fake yagi!" I thought maybe they were serlling them cheap because they were made wrong. With this cteap spuff from china I never know what to expect!

I want to try to put it up this weekend but I think I'll have to wait until next for coax & connectors.

Will post update when I get it on the roof. -Justin

[wingmaker]

Originally Posted by N2UJZ

Ok, thanks guys. I've only built antennas for HF & haven't ever come across a yagi like this one. I didn't think someone would go through the trouble to make a "fake yagi!" I thought maybe they were serlling them cheap because they were made wrong. With this cteap spuff from china I never know what to expect!

I want to try to put it up this weekend but I think I'll have to wait until next for coax & connectors.

Will post update when I get it on the roof. -Justin

Good Luck and Be Careful, Yes and definitly keep us up to date on you progress

LOVE AND BLESSINGS