Theory - Practice - my Quad
The wire in a quad is the actual
antenna; I always use on my home-brew copper-insulated-multi wire: it's cheap, easy to
find and useful.
A 3 mm diameter wire is good also for high power....
Before you must know the following: N8CC made a test to understand what effects wire insulation has on the electrical length of an antenna; he built a 21 mhz dipole with a minimum SWR at 21.1 mhz; than he removed the insulation and when re-installed the dipole has a minimun SWR at 21.5 mhz! (Exactly it was "#14 THHN insulated house wire e di #12 insulated wire"). Keep it in the mind when you build your loops (download this Article).
As far as 21 Mhz wood is OK; you need to varnish it time to time with boat-paint;
Fibreglass is the best, but hard to find and expensive.
PVC is useful as far as 28 or 24 Mhz.
A TV pole (it's strong but weghty).
Aluminum: you can use an old-antenna boom or buy it; 50 mm diameter should be ok even for a 3-4 element on 14 mhz.
You can build a "spider" quad so you don't need boom!
About the shape, circular one has
less load to the wind but you can have some mechanical problem locking the spiders;
the square shape has not this problem, it's weightier than circular one (at same diameter), and I think it's more resistent; rectangular one (with vertical side longer) is more resistent.
BOOM TO MAST ADAPTER
The best will be an aluminum-monobloc-bracket; may be in U.S.A. this will be not hard to find but here....
For a quick solution, an
old-antenna bracket (even from CB) is OK
(see picture 1A);
Home-Brew it is a good idea, just don't build it too weighty.
If you use a circular boom (4-5 cm diameter), you can use the bracket in the picture (see picture 1B); it's very strong!
So now, if you made it to your-self, you can zinc it or just vernish with anti-rust paint.
SPREADER TO BOOM ATTACHMENT
This bracket is very simple (but I don't know if it will work on 3,5-1,8 MHZ HI!);
Buy 4 (one for each element) "L"- bracket, those used for shelf and use it like in picture; as far as 21 Mhz it will work fine (see picture 2A).
The standard approach is to use
steel or aluminum "L" stock machined to intersect in the middle and bolted
together. An additional bar is then bolted between the arms thus created and a muffler
clamp or "C" clamp mounted to the bar.
If well done, this is a very strong assembly. Instead of "L" you can use "T" or "C" stok (see picture 2B).
You can use the above system with pipe instead of "L" stock (see picture 2C).
A square wood-plate on which are locked 4 arms (see picture 2D).
If you have the possibility, use one of the above systems building all arms on the same plane.
Don't worry abot that: often I spoke with russian Om transmiting with quad with simple wood-arms; a local Om here used full-steel arms: their quads performed very well;
The best will be Fibreglass but it's very hard to find here and expensive; fishing rod are less expensive (those are not 100% fibreglass): you need four of them for each element; (see picture 3).
As far as 21 Mhz bamboo is very convenient; I used it for my quad; each pole is 2,5 mt long ($1 each); you can give the bamboo a protective coat of fibreglass that will extend their life indefinitely (Download article on how to do that). I didn't use fibreglass but just marine-wood paint (see picture 3).
The use of PVC or Pvs (for hydraulic or electrical use) is possible, but i suggest do not use it below 21 mhz and do not use diam. less than 30 mm or more than 50 mm (see picture 3) .
Aluminum; yes, i don't mean full
aluminum arms but composite spreaders made from machined PVC insulators at the
element-attachment points. Aluminum tubing is insert into (or over) the insulators 5 cm on
Some years ago i spoke with two OM on 40 mt band: they told me they used for quad "russian pine" (or fir?) and it was very resistent!
FEEDING THE DRIVEN ELEMENT
Impedance in a two element quad should be from 60 up to 100 ohm: here are some system to tune it:
Coax directly to the driven element (with some ferrite-beads or balun) (see picture 4A).
Gamma match on the driven element; in this way the quad is easy to tune but it's a little bit difficult to build (see picture 4B);
1/4 lambda Stub on the driven element; in this case with a R of 100 ohm, using a 72 ohm coax you will have 1:1 (see picture 4C).
Balun-Transformer 2:1 or similar on the driven element;
To obtain a proper current distribution you can use:
Current-type balun that consists of a number of ferrite beads placed over the outside of the feed line, near the feed point ("Super Beads" Amidon, 5 on 40 mt, 4 on 20, 3 on 15 e 10 -from CQ ,July 1994-); the beads were taped up and sealed against moisture;
Linear sleeve balun;
Another simple method of keeping the feed coax shield "cold" for RF is to grid the shield with the antenna attached. If a dip shows up in the band, simply add a few feet of coax to get the dip out of the band, and the shield will be cold, preventing feed-line radiation.
DRIVEN ELEMENTS FED BY ITSEF OR NOT?
A driven fed by itself is the best
way, otherwise you will have a radiation pattern modification (see picture 5), but don't worry, your quad will still work fine:
just you 'll have some problem tuning it!
I suggest you to home-made an antenna switch using common relays (5 or 10 Ampere)(you will find some projects here on my pages. For a tri-bander quad you need only 2 relays!
Quad tuning and adjustment is made in one of the following way on the reflector:
With 1/4 lambda Stub (see picture 6A);
PRO: you can have a fine tune (G,F/B ,S.W.R ecc.);
VERSUS: you must move the regulation-bar on the stub with your antenna up; it's easy to you go up and down on your tower?
Furthermore simmetry is very important in a quad: well, the stub can change it and make the radiation pattern worse; but there is a way: you can build your reflector with the stub, tune the quad and then remove it, you 'll have simmetry back!
With variable capacitor (see picture 6B);
With variable inductor (see picture 6C);
Simply build the reflector 3% longer than the driven element.
HOW TO REDUCE SIZE
Here are some way to make your quad shorter:
With "linear load" (see picture 7A);
With a "Fractal loop" (see picture 7B);