Stacking Yagis

[myspacebarisbroken]

I have been playing with the idea of stacking smaller, identical yagis in order to increase gain without using an insanely long, single yagi. I am thinking about stacking two yagis in the following configuration:

My question is how this array would be connected to my scanner. Do I use a splitter in reverse to combine the signals or do I run cable from only one antenna?

Also what are the official rules in stacking distance, I can't seem to find any straightforward answers. I was thinking about using two cheap HDTV antennas turned 90 deg. about the boom for vertical polarization, but realized that TV antennas are actually two yagis sandwiched together, one for VHF and one for UHF, which explains the "short-long-short-long" order of elements. Would this fact make stacking undesirable as each antenna exhibits two different bands and stacking distance depends on wavelength? Thanks.

 

[radioman2001]

My suggestion is go to the ARRL website and look up antenna designs. A proper way is using a frequency correct splitter, which is both a specific length and has specific impedance coax to match 2 antennas together. the VHF ones we use are about 16in long for 160mhz

 

[myspacebarisbroken]

The frequency range of these antennas are channels 2-69 (54-806MHz), would a general purpose splitter work?

 

[mike_s104]

The frequency range of these antennas are channels 2-69 (54-806MHz), would a general purpose splitter work?

It'll probably work fine. For a few $, try it. I'd run a feed from each antenna to the ground and put the splitter on there then run a single feed into the house. I use the same config with a Scantenna and a Wilson 800MHz yagi and it performs well. Make sure to use a good cable and splitter.

Sent from my ADR6400L using Tapatalk

 

[myspacebarisbroken]

... I use the same config with a Scantenna and a Wilson 800MHz yagi and it performs well...

Are they two separate antennas connected to one coax or are they stacked?

 

[mike_s104]

Are they two separate antennas connected to one coax or are they stacked?

2 different antennas on the same mast.

Sent from my ADR6400L using Tapatalk

 

[bryan_herbert]

http://www.grantronics.com.au/docs/StkYagis.pdf

Stacking TV Antennas

 

[myspacebarisbroken]

Cool, thanks guys!

I saw in an article posted by bryan_herbert that you could stack four yagis and was wondering if that was the limit or could more than four yagis be used in a stack? If so, what are the limits?

 

[jim202]

Cool, thanks guys!

I saw in an article posted by bryan_herbert that you could stack four yagis and was wondering if that was the limit or could more than four yagis be used in a stack? If so, what are the limits?

The limits are what sort of mounting configuration you can come up with. The more yagi antennas you try stacking, the larger of a rotator your going to need.

bear in mind that every time you want more gain, you have to double the number of yagi antennas. like if you start out with one, you need a second yagi to get 3 db gain. Now if you want another 3 db of gain, you will need 4 yagi antennas. if you want another 3 db gain, it will take 8 yagi antennas. Hope you get the point here. It becomes rather impractical after a certain point. All the antennas need to be the same model so the gains stay the same. You can't mix and match yagi antennas with different gains in a stacked system.

Jim

 

[Token]

The frequency range of these antennas are channels 2-69 (54-806MHz), would a general purpose splitter work?

Cool, thanks guys!

I saw in an article posted by bryan_herbert that you could stack four yagis and was wondering if that was the limit or could more than four yagis be used in a stack? If so, what are the limits?

There are no limits as to how many antennas, of any design not just Yagis, can be stacked or combined to yield higher gain. However, mechanical issues start to become very non-trivial when you start talking large numbers of Yagis.

However, you have a problem with your proposal.

Stacked Yagis (or any combined antennas) must be spaced specifically. The spacing will vary with antenna design, but is related to both frequency and boom length (boom length is one of the factors in individual Yagi gain, and gain affects beamwidth, beamwidth affects stacking distances). Note one of the factors is frequency.

As such, your very broad banded application, 54 to 806 MHz, means whatever spacing you calculate and build to will work in one relatively narrow frequency range. Outside this frequency range the antennas might actually reduce overall gain because they will destructively interact instead of constructively.

It IS possible to stack some types of broadbanded antennas, say Log Periodics, but this can be a complex issue. In the case of an LPDA you can stack the antennas so that the booms converge to the front (normally beyond the forward end of the boom) and this will keep the spacing correct for each set of elements in the LPDA. The angles and spacing have to be tailored to the specific antenna set.

T!

 

[myspacebarisbroken]

As such, your very broad banded application, 54 to 806 MHz, means whatever spacing you calculate and build to will work in one relatively narrow frequency range. Outside this frequency range the antennas might actually reduce overall gain because they will destructively interact instead of constructively.

That makes sense! Ok, guess I should just stack yagis for a certain frequency to optimise performance. The reason I'm interested in stacking yagis is to receive some trunking systems that only seem to be receivable at night. I also like the look of a quad-stack config as it reminds me of a WWII portable radar station.

 

[Token]

That makes sense! Ok, guess I should just stack yagis for a certain frequency to optimise performance. The reason I'm interested in stacking yagis is to receive some trunking systems that only seem to be receivable at night. I also like the look of a quad-stack config as it reminds me of a WWII portable radar station.

Not only WW II radars used Yagi arrays, they are still used today. The Russian 1L13-3 Nebo, 1L119 Nebo, RLM-M, R-18 Spoon/Knife/Fork Rest series, Chinese JY-27, and YLC-8 all come to mind, but I am sure there are a lot more out there. Some pictures will be in this paper Russian / PLA Low Band Surveillance Radar Systems (Counter Low Observable Technology Radars)

T!

 

[prcguy]

In addition to Tokens excellent comments, a high quality combiner (backwards splitter) or narrow band phasing harness will give the least amount of combining loss and will get you closer to the theoretical 3dB combined gain.

I've measured TV splitters with as much as 5dB loss which will eat up most of you're gain, you really want to find the proper combinng method. I dont believe most TV splitters are designed to provide signals perfectaly in phase which is required for combining.

The lengths of cable between the combiner and stacked antennas must also be the exact same electrical length. If your using a phasing harness it will be a specific length for the frequency in use or with a combiner the cables can be any length as long as they are the same.

When your done stacking two TV antennas you will have exerted way more effort than finding a single antenna with more gain.
prcguy

 

[myspacebarisbroken]

I'm not going to stack the tv antennas, way too much work! I'm just going to build some homade yagis and stack them.

Any suggestions as to combiners or phase harnesses? Remember, I don't have a million dollars!

 

[Token]

I'm not going to stack the tv antennas, way too much work! I'm just going to build some homade yagis and stack them.

Any suggestions as to combiners or phase harnesses? Remember, I don't have a million dollars!

You can build the complete phasing harness yourself for relatively low cost, if done correctly no combiner is required, the phasing harness is the combiner. A quick Google search for Yagi phasing harness should yield a few answers.

Keep in mind that stacking antennas and using a phasing harness will narrow the usable bandwidth a bit. You can probably not expect more than about 10% of bandwidth maximum, and probably less than that. So, if you build a pair of Yagis for say 155 Mhz you can expect maybe up to 15 MHz of usable bandwidth, a pair of 450 MHz Yagis might yield as much as 45 MHz bandwidth, but as I said before possibly less also. Beyond that 10% the antennas will still receive, but the performance drops off pretty steeply and in transmit use the SWR rises even quicker. Using a combiner, or power divider, will broaden the bandwidth from the transmit aspect, it is possible to get 25% or more from a properly built power divider, but unless your Yagi design is pretty broad banded the antenna itself will likely be the limiter.

Building a phasing harness for two Yagis is pretty straight forward.

From each Yagi feedpoint a 75 Ohm coax (say RG-11, RG-59, or RG-6) is used. Each of these coax pieces are odd multiples of quarter wavelengths ( 1/4, 3/4, 5/4, 7/4, etc) electrical length at the design frequency. The electrical length means the actual physical quarter wavelength (calculated for free space) multiplied by the velocity factor of the specific coax.

At the end to be combined (far end of each 75 Ohm coax) the coaxes are connectorized and attached to a simple T. From there any length 50 Ohm coax can be taken to the receive/transmit location.

T!

 

[majoco]

Where do you get these yagi antennas that work from 54 to 806MHz? A dipole that is a quarter wave long and it's reflectors/directors has a pretty narrow frequency bandwidth - at any other than it's design frequency it'll be anything from a short circuit to open circuit. The lengths of coax in the phasing harness is extremely frequency critcal too so joining two different antennas together with simple coax is doomed.

Sure, you can phase together multiple identical antennas for a narrow frequency band but the practicalities of it are pretty difficult without vector voltmeters and line stretchers.

 

[myspacebarisbroken]

Hi, majoco, the yagi with the frequency range of 54-806 MHz is just a standard TV antenna, but you have to turn it 90 degrees for vertical polarization.

Thanks for your concern, but I am not determined that the antenna be perfect, afterall, I have just recently become interested in building antennas. I just thought I'd try my hand at a quad stack and see what happens. I'll be happy even with a slight performance increase over a single yagi.

How are things in NZ?

 

[wildbillx]

Can someone recommend a splitter/ combiner that I can use for a wilson yagi 800mhz and a diamond discone. I want to run it into 2 scanners.