I have made my first biquad antenna and it works rather well. Now I want to compare different antenna designs and therefore I use N-connectors so I can swab them. Since I have to make them I wonder if it is useful to make a balun in the feed of a biquad? On the internet I found one guy who have made such a device but no details are given. Why is it some antenna's have baluns and others have not?
most cases, it can be as simple as a 1/4 wave sleeve around the coax.
The purpose of a balun is to prevent the coax cable from radiating. However, in order for the coax to radiate, there would need to be some radiative component along the axis of the coax to induce current in the coax. A symmetrical biquad doesn't have any such field component. Therefore, a balun is not needed.
However, things get a bit different when the balun is used to feed an offset dish. The field is no longer axially symmetrical, and the coax is in the reflected field from the dish. Therefore a balun is probably a good idea with a dish.
In general (with lots of exceptions), when the coax cable is in the antenna field, you need a balun. The real question is how much difference does it make? Most of the effect is in reducing VSWR, not increasing gain. With relatively low gain antennas (i.e. biquad with
10dBi gain), it probably might improve things perhaps a few tenths of a dB and is not worth the effort. However, with higher gain antennas, such as a dish, the lack of symmetry caused by a radiating feed could easily distort the pattern, create sidelobes, boresight errors, and VSWR.
Also, some antennas are best constructed with 200 ohm feed points (i.e. Franklin antenna).
a 4:1 balun solves the balance problem and the impedance transform problem at the same time. You may therefore see a balun on some symmetrical antennas that don't normally require one, but where the impedance transformer action makes one convenient.
Yep. That will work if the grommet is a good insulator. It also needs to be soldered to the coax shield at the end away from the quad. Soldering to the common aluminium foil wrapped coax is not fun. So, he has to use RG-213 or something similar.
Incidentally, his gain estimate is wrong. No reflector is 100% efficient. At best it's about 50% or 3dB less gain. Figure on 11.4dB maximum gain (ignoring losses).
shows 11.1 maximum.
Maybe. However, anyone making as big a mistake as 3dB in gain, not posting sufficient construction details, and not including an NEC model, is somewhat lacking. The tiny wire extension between the coax shield and the quad elements also makes me suspicious. Also no photo or test results of the finished product. It's a good idea but there's quite a bit missing. On the other hand, G8OTA is the keeper of
That might explain why he did it :-) I planned to use the biquad as a feed for a dish too and I am still searching for the best feed for a dish. This looks interesting too
I made the balun like that, now I only need to find some thin copper or brass plate for the dipoles. I wonder what the difference is between these dipoles and "normal" ones like here :
Beside that I've made a N-connector for an cantenna too, a cantenna might be a good simple solution for quick testing and hopefully gives me enough signal here. I am expecting to receive some connectors tomorrow then I can hook up the linksys router you suggested some time ago. This router seems to work fine with the DD-WRT firmware, I managed to use it as a client with the own antenna's only, I do not have a suitable connector for the linksys yet. To be continued........
I noticed they don't show a double biquad. Also, Jeff pointed out in previous posts that you need to property illuminate (my word) a dish. I don't recall his phrasology, but there were good links provided.
Jeff had provided a link on how to use a helix to illuminate the dish. You control the beamwidth with the number of turns.
The advantage to the helix is it is a wide bandwidth antenna. Not that you need wide bandwidth, but rather you will not be on target with your home brew antenna, and few people have the tools to tune their antenna. Wide bandwidth means whatever you build is likely to cover your intended band. [I do this with log periodic antennas, i.e build them wider just to conver manufacturing goofs.] The disadvantage is the impedance matching.
The circular polarization is part advantage and part disadvantage. There is a 3db loss due to the circular polarization, but at least you don't have to worry about having the proper polarization (i.e horizontal or vertical) since the circular polarized antenna will speak properly to either antenna.
The helix doesn't need a balun as it is already unbalanced.
I remember that, I've read them all ( I think ) but I am still not sure what is the best solution for illuminate a dish. As far as I can see most commercial dishes use a dipole for it, if that means it is the best solution, I don't know :-)
The main online reference on dish antennas is at: |
I'm not a big fan of helical dish feeds because they're a bit too complex to build. However, the ability to adjust the feed beamwidth by simply adding or removing turns is a big win for illuminating a random dish antenna. To determine the necessary beamwidth, just take two pieces of string and connect both from the dish focus to the edges of the dish. Measure the angle between the pieces of string. That's your target -3dB beamwidth.
(see various AO-40 antennas).
The reason that some dish antennas can use a dipole antenna is that they are receive only. For example, the common MMDS dish and feed: |
receive, the feed can be overly wide and not affect the receive gain much. It will cause horrible side lobes, but most people don't notice those. However, in transmit, having the feed overspray the dish causes considerable loss in xmit gain. Incidentally, this is why cramming a USB dongle into a random dish results in a spectactular receive gain (as shown by Netstumbler or Kismet), but has problems communicating because the xmit gain is lower.
As for biquad dish feeds, this one looks well built.
that's an awfully big feed for such a small dish. It would be more suitable for a larger dish antenna. A helix or patch would be even larger.