Stacking Yagi Antenna's

[NJ-Radio]

Has anyone tried stacking yagi antenna's to achieve additional gain?
What was the results and was it worth it?

I have a homemade Yagi cut for 485Mhz to monitor NYPD from south Jersey. It works
well but could use some additional gain for the weaker precincts. I am planning on
making another yagi so I can tie them in together but would like to hear from others
that have tried this and if they did achieve extra gain making it worthwhile.

Any tips that can be provided as well would be great.


Thanks

Brian

 

[zz0468]

Done PROPERLY, you'll get a bit less than 3 db additional gain by stacking yagis. Depending on what you're starting with, and what you actually need, you may be better off just getting (or building) a better yagi.

 

[DPD1]

There's many hams that have done it... I've done it and I personally don't think it's worth the trouble. If you were doubling your performance, maybe, but you're not. I think you'd be better off spending the time on a tower or something probably.

 

[NJ-Radio]

I didn't expect to double my signals, although that's be nice.
If it came up at least 1 notch on the signal meter that would be good.

My current yagi is tuned pretty well so I don't think I can do much else
with it. It has 10 directors. I guess I can add a few more making it a longer
boom but that gain would maybe be 1 more db and the extra weight would be
harder to keep it from dipping on the mast (without a non metallic brace which would be an
eyesore).


Brian

 

[BuffaloJustice]

If I can find it there is a good article on stacking antennas. The basic idea is to stack them one wavelength apart on the frequency you want to use. This will net the most gain on that freq and a little less on the others in the band. You will also need to feed them in phase or you can reduce the gain considerably, in fact done perfectly incorrectly you can cancel one antenna with the other. And unless I'm missing something, a near 3db increase IS nearly double. Both antennas would have to be perfectly identical, spaced correctly and be fed correctly in order to get that number but it could be worth it.

Disclaimer: I am new to antenna design and basing all opinions on reading I have done in regards to the subject covered. I may not have correct info or may be applying what I have read incorrectly. As such, any constructive information is welcome as I would like to learn as well.

 

[NJ-Radio]

Thanks Buffalo.

Yes, there are a few good articles I've ran into and they do describe how to do it properly.
I just wanted to hear from someone that actually tried it and what they're results were to see
if it's worthwhile trying it.

A 3db gain would be far from doubling. My current yagi has an approximate gain of 12db.
Doubling would be 24db.



Brian

 

[BuffaloJustice]

Okay, yea I got ya. I was thinking elsewhere.

 

[W6KRU]

A 3db gain would be far from doubling. My current yagi has an approximate gain of 12db.
Doubling would be 24db.



Brian

A 3dB gain is a doubling of signal. Every 3dB is another doubling or put another way a 6dB gain is a quadrupling of signal. If your yagi has 12dB of gain and you do something to double the signal then that would be 15dB of total gain.

Now if you're still with me, a doubling of gain would give you 24dB of gain.

 

[zz0468]

A 3db gain would be far from doubling. My current yagi has an approximate gain of 12db.
Doubling would be 24db.

Wrong!

When speaking of antenna gain, you're dealing with RF power - albeit very small amounts of it. A doubling of power is a 3 db gain. So, doubling the gain of a 12 db gain antenna, as in stacking, would create an antenna array of 15 DB gain, not 24.

But the reality is, stacking gain will be less than 3 db.

 

[zz0468]

A 3dB gain is a doubling of signal. Every 3dB is another doubling or put another way a 6dB gain is a quadrupling of signal. If your yagi has 12dB of gain and you do something to double the signal then that would be 15dB of total gain.

Now if you're still with me, a doubling of gain would give you 24dB of gain.

You're half right. The first paragraph is correct.

The second paragraph is wrong. Decibels are logarithmic. Factoring gain, as in doubling it, is not.

A device with a power gain of 2 has 3 db of gain. An pair of stacked antennas that double the gain of a single antenna has a power gain of 2 - or, 3 db.

 

[BuffaloJustice]

See, this is what makes my head swell, in a painful overload sort of way not the proud I'm so awesome sort of way.
That was my point with the 3db, theoretical, gain gotten from a stacked array doubling the signal (which would make it a 15db array.) Then it seemed as if he wanted to double the total gain figure.

So, am I right that if his current antenna netted him 16 units (a made up number), then a stacked system, adding 3 db (theoretical of course) would take him to 15db total and would net him 32 units? And also that "doubling the gain total" to 24 db net him 256 units? I haven't torn into the antenna guide book much yet, so I get confused with antenna theory. easily.

 

[Josh]

I have a VHF Yagi stack set that does pretty well, horizontally separated by 5/8 wavelength.

an additional '3db gain' or doubling of the signal will be obtained by adding the second 10 element antenna, doubling the number of elements to your current antenna, or doubling the height above ground.

Either way, it will be noticeable, but not a whole heck of a lot, it won't be night and day, but it will make a beneficial difference, if nothing more than helping overcome cable loss, etc.

Perhaps the cheapest investment could be in better feed line?

-Josh

 

[n5ims]

While this isn't totally accurate, hopefully it'll give you some idea of what's going on.

DB is similar to powers in math. Instead of stating x to the first power, they say 0 db gain, x squared would be say 2 db gain, x cubed would be 3 db, etc. Assuming x is 2, 1 would indicate 2, 2 would indicate 4, 3 would indicate 8. DB isn't quite as easy as this, but the general idea is similar. That's why doubling the power is 3 db so an doubling 10 db would make it 13 and not 20 as you may expect.

The antenna theory books will have the charts and examples to help explain it is real terms, but hopefully this little math theory will help you understand why things don't look like they add up, when in reality they do.

 

[kb2vxa]

You guys sure love to show off your technical skills with pointless arguing so lets get TO the point. Stacking won't buy you nearly as much as improving the antenna itself simply by lengthening the boom and adding as many directors as you like within physical limits, there is no electrical limit. Just measure the existing ones and make the rest all the same length and spacing. Not only will this give you considerably more gain but will narrow the beam width and reduce side and rear lobes considerably giving you greater directivity, an added advantage stacking will not accomplish.

 

[BuffaloJustice]

So you're the one that keeps fixing the dang thing while we're trying to figure out what the best answer is! Stop it... Hey I said stop! We're trying to figure out what's wrong with it!

 

[prcguy]

Lengthining the boom is not so simple, the entire design changes and the spacing of the original elements will not be optimum. A general rule of thumb for yagi gain is double the boom length to get 3dB gain. That's not practical in many cases and adding a second yagi of the same size is usually easier than rebuilding an existing antenna.
I went the long boom route for 2m and assembled an M2 yagi with a 33ft long boom. Then I realized it will hang over my neighbors yard by 8ft and have not installed it yet. Dual 15ft yagis would have been a better choice and now my M2 yagi is a giant clothes line for the wifee.

You guys sure love to show off your technical skills with pointless arguing so lets get TO the point. Stacking won't buy you nearly as much as improving the antenna itself simply by lengthening the boom and adding as many directors as you like within physical limits, there is no electrical limit. Just measure the existing ones and make the rest all the same length and spacing. Not only will this give you considerably more gain but will narrow the beam width and reduce side and rear lobes considerably giving you greater directivity, an added advantage stacking will not accomplish.

 

[NJ-Radio]

Ok. As expected, I got an assortment of mixed information.

I don't care about learning the mathematics involved, except for what's needed when designing the stacked pair.
Just looking for people who tried this - particularly in the UHF band - and what the results were.
As in, a single S-meter improvement, no improvement, worse....things like that.

And, as for those who keep saying just build a longer single yagi - that is absurd!
I believe to achieve a 15db single yagi it would need to be like 30 or 40 feet long - or something to that effect (don't give me the math on this figure cause I dont care).
Stacking is the BEST option - I already know that from reading. I just need to try it I guess to
see the results myself. With all the radio guru's here I'm surprised I didn't get more feedback on this.

 

[BuffaloJustice]

Yea, that happens here some times. Just follow the numbers from a good plan on the net and see what happens. I'd be willing to bet that it does the trick and if not you got to learn something. I built a copper j pole from an internet design and copied it to the tee, when we hooked it up to an analyzer it was pretty well dead nuts. (145.010 and 1.1 at the edges)

I know some one else already said it but you may get some additional gain, really less loss, with better feed line. What are you using and how long is the run?

 

[NJ-Radio]

Hey Buffalo!

Actually my cable is pretty good - it's LMR400. I'm also using a Ramsey broadband preamp which helps alot to.
I'm just looking to see if I can get a little more because the Bronx NYPD comes in a little weak and a few other precincts. Overall I'm thrilled that I'm able to get them at all from 60+ miles away.
I do enjoy antenna building and had fun making this yagi so I may just try it and experiment.

Brian

 

[BuffaloJustice]

Yea, so you're not gonna gain anything out of upgrading the cable. You're doing well for 440 and if you already get those precincts you should see an improvement. Let us know how it goes.