Stacking Yagis
GM7PBB
I would like to stack to 2 x 50 Mhz 6 element Yagis, What would be the
best length for the phasing stubs of 75ohm coax to each Yagi from tip to tip
of the N-Plug. Look forward to hear your comments.
Many Thanks in advance
John GM7PBB
John, Julie & Owen :-)
Eoropie
Isle of Lewis
www.gm7pbb.co.uk
Bill Tippett
> I would like to stack to 2 x 50 Mhz 6 element Yagis, What would be the
> best length for the phasing stubs of 75ohm coax to each Yagi from tip to tip
> of the N-Plug. Look forward to hear your comments.
Your first consideration should be vertical stacking distance which
is a function of boom length. A rule of thumb is to stack at the square
root of the boom length in wavelengths...i.e. if the boom is .81
wavelengths, vertical stacking distance is .9 wavelengths. However, the
most accurate way to determine this is with a modeling program such as
EZNEC or YO. Once you have the correct stacking distance for your
specific antennas, you then need to determine how much coax you need to
reach the midpoint of the antennas. Assuming you have a matching
network at the feed point (at the midpoint), you only need to make sure
the feedlines are the exact same length. Check
http://www.arraysolutions.com/products.htm for the StackMatch and
StackMaster products which will both match two 6 meter Yagis.
If you are attempting to use 75 ohm feedlines as 1/4 wave
transformers (assuming 100 ohms at the antenna and 50 ohms at the
connection of the two phasing lines), the feedlines should both be the
same odd multiple of electrical 1/4 wavelengths. Assuming you need
about 1 wavelength stacking distance, your feedlines should each be 3/4
wavelengths which will give you enough physical length to run the coax.
For 50.1 MHz resonance and assuming a velocity factor (Vf) of ~0.8 for
foam-type coax, each line should be (983.56/F) * .75 * Vf. In your
case, this would be 983.56/F = 19.63 ft (one electrical wavelength)
times .75 times .8 = 11.78 ft for each feedline. HOWEVER, this assumes
you know the EXACT velocity factor for your coax. If you don't, you
will need to use some sort of network analyzer (MFJ-259B works well) to
determine the exact electrical 3/4 wavelength. This method also assumes
you match the feedpoint impedance of both antennas to 100 ohms so you
will need an analyzer to do that also. You must also make sure
everything including physical balun alignment is identical for each
antenna.
The StackMatch device mentioned above works well for non-resonant
feedlines and I am very pleased with my 3-stack of 6 element Yagis for
28 MHz. Here are pictures of my system:
http://www.arraysolutions.com/Users/w4zv.htm This system now holds USA
records for both CW and SSB in both the CQ WW and CQ WPX Contests so I
am very pleased with it!
73, Bill W4ZV
Bill Tippett
>Check
> http://www.arraysolutions.com/products.htm for the StackMatch and
> StackMaster products which will both match two 6 meter Yagis.
I should have added that the Mini StackMatch should work for your
case. The HF version is $125 but you should check to see if a special
version is needed for 50 MHz as is the case for the standard
StackMatch. Good luck!
73, Bill W4ZV
Bill Tippett
> If you are attempting to use 75 ohm feedlines as 1/4 wave
> transformers (assuming 100 ohms at the antenna and 50 ohms at the
> connection of the two phasing lines), the feedlines should both be the
> same odd multiple of electrical 1/4 wavelengths. Assuming you need
> about 1 wavelength stacking distance, your feedlines should each be 3/4
> wavelengths which will give you enough physical length to run the coax.
> For 50.1 MHz resonance and assuming a velocity factor (Vf) of ~0.8 for
> foam-type coax, each line should be (983.56/F) * .75 * Vf. In your
> case, this would be 983.56/F = 19.63 ft (one electrical wavelength)
> times .75 times .8 = 11.78 ft for each feedline. HOWEVER, this assumes
> you know the EXACT velocity factor for your coax. If you don't, you
> will need to use some sort of network analyzer (MFJ-259B works well) to
> determine the exact electrical 3/4 wavelength. This method also assumes
> you match the feedpoint impedance of both antennas to 100 ohms so you
> will need an analyzer to do that also. You must also make sure
> everything including physical balun alignment is identical for each
> antenna.
Oops...too early in the morning! This system works for matching
two 50 ohm antennas to 50 ohm coax at the tee connector. I was thinking
about the 2:1 transformer backwards. It actually converts the 50
antenna impedance to 100 ohms at the end of the 3/4 wave transformer and
then connecting them in parallel with a tee yields 50 ohms again.
Therefore the feedpoint impedance of both antennas must be matched to 50
ohms...not 100 ohms! The lengths are still correct as above.
73, Bill W4ZV
Dennis Kaylor
i have a question on stacking for you
do you know the proper stacking distance to stack 4 horizontal pol 10 ele
quads the boom lenght is around 12 foot
thanks
Ian White, G3SEK
>Hi All
>
> I would like to stack to 2 x 50 Mhz 6 element Yagis, What would be the
>best length for the phasing stubs of 75ohm coax to each Yagi from tip to tip
>of the N-Plug. Look forward to hear your comments.
>
leg of the power divider to bring the 50 ohm yagis up to approximately
112 ohms, which you will then parallel for a reasonable match to 50 ohm.
If you're going to do that, then make it *one* quarter-wavelength in
each leg of the power divider, and do the rest of the run out to the
individual yagis in matched 50 ohm coax.
*Don't* use multiple quarter-waves of 75 ohm - the phasing becomes
insanely sensitive to small errors in length, terminating impedance
and/or velocity factor. (Ignore what the books recommend about using
multiple quarter-waves - it's all based on the same ages-old information
that was never actually thought through. A leading UK 2m moonbouncer
just came unstuck with exactly that problem, and was horrified when he
modelled it to find out what was really happening.)
The lengths you need would be 99cm of solid-polyethylene 75 ohm coax.
There's no need to be more accurate than that on 6 metres, but make the
two lengths identical. Then use two identical lengths of 50 ohm feedline
to get to the yagis - those lengths are not critical, just make them the
same, and however long the longer one needs to be.
At 6m you don't have to use lots of N-connectors - just make the best
soldered splices and T-joints you can, and after you've tested
everything, cover the joints with self-amalgamating tape. There are some
ideas about splicing coax in my 'In Practice' column for June 1998.
W4ZV has talked about the virtues of both/lower/upper (and even
antiphase) switching, and that's a very nice facility to have. Besides
the Stackmatch route, the ARRL Antenna Book has good information in the
section about stacking HF yagis (but once again, *ignore* the section
about stacking VHF yagis using multiple quarter-wavelengths of 75 ohm
coax).
The other thing you need to think about is stacking distance - there's
information on my web site, and especially the page from OH1ZAA which is
specifically about 6m.
>John, Julie & Owen :-)
>Eoropie
>Isle of Lewis
I'll be on the Isle of Mull in August - catch you on 6 with your big
signal!
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
Bill Tippett
> do you know the proper stacking distance to stack 4 horizontal pol 10 ele
> quads the boom lenght is around 12 foot
Dennis, I assume you are talking about 2 meters...do you mean vertical
or horizontal stacking? If vertical, the formula I gave would indicate
about 1.33 wavelengths or a bit over 9'. If horizontal stacking, check
out the ARRL Antenna Book which has a chart I recall but don't have
access to it at the moment.
73, Bill W4ZV
Dennis Kaylor
actually it will be in an H frame so it will be vertical and horizontally
stacked 2 on 2 forming a large H with each antenna on a corner
if it where yagies i would know what to do just never done quads before and
dont know if its any differnt
thanks