Theory - Practice - my Quad
MY QUAD (2 EL. FOR 10-12-15 MT)
I built two quad (one to me, the other to Mario, ik7zcq).
I used "+ "
("Diamond") configuration which gives just 0,1-0,2 db more in the gain than
"Square" configuration: I built it in this way only for mechanical reasons. I
used square-aluminum boom 30*30*1 mm 180 cm length; into it I put a steel pipe just to
make it stronger; the mast to boom
attachment comes from a CB antenna ($6)(see picture 1).
The spreader to boom attachment (one for each element) is home-made using a square-steel bar (35 cm length); on the tips there are 4 perpendicular pipe (25 mm diameter) where you'll insert poles (see picture 2).
Poles are 5 mt length (21 mhz); I use four bamboo poles for each element ($1 each); each pole is 240 cm length so I made it longer using in the lower part a short aluminum pipe (25 mm diameter, 100 cm length).
I gave the poles a protective coat of boat-paint and then I covered it with PVC ribbon (thanks to Jan, G0KRL for info).
Loops are on copper-insulated multi-wire (1mm for my quad, 3mm for Mario's one).
Here are formulas for loop length:
L=Length for one side.
I never spent time tuning it (it's "no problem, no tune", and works fine!) but I know working some hours I can optimize Gain or F/B Ratio ecc.
I fed driven elements with 3 (one
for each band) 1/4 lambda stub using TV 75 Ohm cable; on the Mario's quad i used RG 11
useful for high power;
over the outside of the feed line, near the feed point, there are some ferrite beads taped up and sealed against moisture: that's my balun (see picture 3); on the boom there's also the rele' box for antenna switch (see picture 4); then a common 50 ohm RG 213 goes down on the shack.
I suggest you do not solder the stub directly on the loop, but using a SO239: here it's still working fine but will be better how i suggest.
The weigh of my quad is about 15 Kg.
HOW DOES IT WORK?
Oh guys! It works very fine! My
quad is only 8 mt up, but it never performed worse than a 3 element yagi (sometime I hear
what yagi doesn't; I don't know why european stations give me better reports than mine (If
I give them 5/7 they give me 5/9, 5/9+); with Dx station reports are the same. The same
occur with F/B Ratio: it's about 15 db with european stations, more with Dx.
Using the quad I forgot my antenna-matcher: SWR is less than 1:1,6 at the end of the band;
I think home-brew this antenna is easy: everybody can do it! I spent less than 100$ and I have a very fine beam; may be this summer I'll add 18 e 14 loops but this is an other story
73 de iz7ath, Talino