Q:...is it necessary to include a balun?
A:... NO it's NOT necessary for this antenna because you are
generally
Receiving and not Transmitting.
Want to know more... I did make a balun for this antenna by
winding 6 rounds of coax around a 2" PVC pipe.
Held it
with electrical tape and slid the PVC out. I bound this 'roll'
with electrical tape to create my 'ugly balun'.
It's not a separate piece of coax! The "roll" is made from the
longer transmission line coax and is placed close to the Driven
Element.
Q:...My USB dongle has a cable with a large plug on the other
end. I assume I need to also buy a large plug to connect the
coax to this little lead?
A:... Yes. It sounds like you have a patch lead so use whatever
fittings or adaptors YOU need to achieve a "join". Each time you
introduce another adapter you degrade your signal a little more,
so try to keep them to a minimum.
Q:...Can you clarify where you put the lugs onto the end of
the Coax RG58. I presume that one lug goes onto the single
middle wire and the other onto the multiple shield wires. Am I
correct?
A:... Correct. Cut away about 25mm of the black coax outer
plastic and pull some braided shield mesh to form it into single
thick wire. Then carefully remove the white plastic dielectric
to expose the centre wires and twist them into a single wire.
It's a fiddly process but now you have 2 separate tails to work
with and each tail is soldered to a lug.
The lengths you need will vary depending on where you drilled
your Coax entry hole and the 'angle' you need to achieve.
Q:... You suggest cutting the beam through the hole for the
driven element. Is there a reason that the cable ends need to
be attached to the very inside ends of the driven element -
could I attach the coax to either side of the driven element
outside the riser?
A:... Where you attach the coax alters the electrical length of
the Driven Element. (I know the physical length remains the
same). To maintain the required electrical length of the Driven
Element we need to have coax tails connected as close to the
centre as possible.
In practical terms try for a 1mm gap between the two halves of
the driven element then put the two holes/lugs as close to the
ends as practicable but don't allow the two lugs to touch.
Q:... I could only get 12mm Aluminium tubing not the 10mm
mentioned. Do I need to adjust the spacing to allow for the
extra 2mm in width of each element?
A:... The technical answer is Yes... when you alter the diameter
of the material it will change the specs but the alteration
would be fraction of a millimetre. Most people wont be anywhere
near that accurate in their building to begin with so I
personally wouldn't worry about it. Just go with the
measurements above.
Q:... You suggest 900mm length, yet the spacings of the
elements from the reflector add up to 2,361mm. I must be
missing something!
A:... Yep. You missed the keywords
Element spacings: from Reflector. Those
measurements are not ADDITIONS.
You take the centre of your Reflector and each one is measured
from that point. So Driven Element is 74mmm out from the
Reflector: Director #1 is 124mm out from the Reflector and so
on... Until finally Director #6 is 670mm out from the Reflector
Your boom can be anywhere from about 700 mm onwards depending on
how you're going to mount it. In my case a 900mm riser was a
convenient length, with enough sticking out the back end to
mount it.
Q:... Coax Question - Is RG213 better than the RG58? I read
that it has a lower loss per metre. Even better than that is
the LMR400 which has roughly half the loss of RG58?
A:... RG213 is better than the "cheap" RG58 coax and good to see
you researching. RG213 is a thick coax and more difficult to
work with as you have to finally connect to the tiny FME antenna
fitting on the modem and the more adapters and patch leads you
add will degrade your signal, so it's a trade off.
Working with a thinner coax gives more flexibility and there are
high quality RG58's on the market but you have to search around
for them. Jaycar generally sell the cheap coax with markings
like /R or /CU on them.
My Coax is Cellfoam Low Loss RG58 Type RFI CAT. No 9001 which is
rated as good as RG213 and if you're serious about finding this
quality look for the "Installer" in your town who puts radios in
Ambulances, Fire Trucks etc... They should have high quality
stuff as it's a required spec for emergency vehicles. They may
sell you 20 or 30metres, it's worth a try.
Q:... Can the Driven Element have wood inside? I realised that
a standard wooden pencil fits perfectly inside the aluminium
tube and makes the D.E. strong.
Another question; To change the polarity to vertical I only
have to put the antenna with the directors in vertical
position.
A:... A wooden pencil to keep the two halves apart and
strengthen the whole thing is a brilliant idea as wood is also
an insulator.
And, yes you are correct. If your nearest tower is Vertical you
place your antenna so the aluminium cross pieces are pointing up
and down. If your tower is Horizontal you place them crossways,
just like looking at the horizon.
Q:... I'm asking about a Yagi antenna covering 850mhz to
2100mhz frequencies used in the Philippines. Some telecoms
here use 850/2100MHz. I made your Yagi design, its 100%
success to my broadband connection, could you pls give me the
design so I would try?
A:... I don't build 2100MHz antennas so I can't recommend
anything but.. If you check this Whirlpool.net.au page there is
a list of Yagi's for various frequencies. Once you find the Yagi
specifications you have my basic design/build ideas and should
be able to make something just for the 2100mhz signal.
Q:... Do you have to tweak the Elite Modem so it knows you
have an external antenna.
A:... There was no option to tell the modem I was using an
external antenna until I upgraded the firmware for the modem.
WOW an option then appeared asking if I was using an external
antenna.. that was a surprise hahaha.
Q:... The 'telcoantennas' website says a Yagi provides
upwards of 12db gain. What gain would you expect from this
design?
A:... Correct a Yagi = about 12dB gain (improvement) over a
dipole antenna.
Don't worry too much about it, there are some basic rules like;
the more Directors you have out the front of your antenna
increases Gain but it narrows the bandwidth of the antenna...
too technical for this project.
Want to know more... a simplistic explanation of how a Yagi
works: Imagine a Vertical antenna, your signal is going out in
all directions... a lot of your 'energy' is wasted if you didn't
want it to go up, and out those other directions.
Professor Yagi & Shintaro Uda discovered they could harness all
those outward radiating radio waves into one direction. The Yagi
beam doesn't allow the signal to go out the back or to the sides
but rather it goes in an elongated egg shaped 'lobe' in the
direction that the beam is pointed.
And it works in reverse, the forward directors pick up a signal
and effectively amplify it they pass it backwards from director
to director until it strikes the driven element and it becomes
of use to us.
The reflector at the back simply halts the process and bounces
anything that hits it back to the driven element as well.
Q:... Would that NextG yagi antenna be suitable for mobile
phone (next g 850 band) as well as NextG USB broadband? Both
my phone and the elite usb modem have external sockets and I'm
able to order the patch cables for both so everything should
work fine hopefully
A:... Yes, the frequency is the important thing and if your
devices will accept an external antenna you can connect the Yagi
to capture NextG/3G Frequency. USB dongle modems sometimes have
external antenna ports but some do not.
Q:... looking at the photos of the Yagi it appears that the
directional bars are the ones to be either horizontal or
vertical. Is that correct?
A:... Yes, when we talk about the "plane" of the antenna being
either horizontal or vertical we are talking about the cross
pieces of aluminium - the elements.
Telstra/Bigpond mount their antennas either Vertical or SLANT on
their towers..... you check your nearest tower (from the ACMA
records) and position your antenna to match the tower they are
pointing at.
Remark: This is an email from "M" regarding his search for
NextG Antennas... made me smile
Below is a reply that I sent to a Retail outlet this morning
after inquiring how much for their 16.5Db Yagi & good cable,
delivered to Mullaley NSW. Answer $194.70
Thank you for the speedy reply. While I was looking on the net
for antennas I came across vk4ion.au/nextg_yagi.html which
details a home made yagi antenna with instructions. I had
nothing to loose so in under an hour I was replacing my store
bought 12Db with this PVC & aluminium tubing nightmare.
Previously I was getting low signal strength @ 0.36 to 0.640KbS
linespeed, and now 2.34MbS. Well I couldn't be happier. Total
Cost about $6. But once again thanks for the reply. Your
faithfully M.
In all fairness I must admit that my effort looks like a shower
of s#*t but it works a treat for the 76Km straight line. I
didn't have 10mm tubing for the elements, I used 12mm and the
riser was 3x offcuts of 50mm PVC. The cable was supposed the be
the best (Industry Standard) I was told, thin RG58. All I did
was take the U-Beaut shop Yagi antenna down and put up the hours
labour. I think I will go back to the drawing board and this
time do a professional job on it, including replacing the RG58.
Reply:... Hello M. Thanks for the contact which made me smile.
Yes, we are trained to believe that retailers know best and we
do need them to help those who can't clamber on the roof. But
having said that, I want the Commercial guys to learn their
craft and don't just take people's money. Your funny anecdote
may inspire others and btw GREAT speedtest results too, thanks
again for sharing.
Msg from James: I've now built a few of them! My biggest
advice to those building it would be to get good coax, I ended
up using RG213 as I was having trouble sourcing good quality
low loss RG58 coax, this was a great improvement over the
cheap RG58 that I originally got from jaycar although it is
tricky to work with such large cable. James, Stanthorpe Qld
Reply:... Hi James, Thanks for feedback. Yes... there is no
substitute for quality coax and it's annoying when a salesman
says... "they're all the same mate" haha
When people take this leap of faith to build my antenna it's
understandable they'll user cheaper coax to keep the cost down
in case it doesn't work.
But if it does work replacing the coax will give that boost as
you've outlined.
