Antennas are the most misunderstood subject when it comes to radio. If required, an antenna can be put together that, although appears correct in theory and design - including matching perfectly at the required frequency, is nothing more than a RF sponge absorbing almost all of the power injected into it with little being sent in the direction required.
Further to this, Low Power Radio offers little opportunity to waste any of this precious power and the radiation efficiency of an antenna is of paramount importance if reliable links are to be achieved and maintained.
Sadly there are many "jokers" in the antenna supply arena which makes it difficult to assess whether one is buying a good radiator or a sponge. Personally, I have a few criteria that I use and hopefully this will assist in deciding on your supplier - as well as ensuring the antenna is up to the job!
1) Price is not an issue - This may seem like a stupid statement but if one looks at the repercussions of employing "dicky" peripherals then price of the radiation equipment starts becoming insignificant. An analogy is take a Ferrari, would you put standard 185/70/14 tyres to it (or whatever size the rims are). Here you have a machine capable of exceeding 250km/h but the tyres are not allowed to handle beyond 180 safely. Secondly, they are not designed to steer the frame of this low lying car round corners unless done so sedately.
With this analogy to ride on (pun intended) why would you want to use a piece of serious equipment (your radio) with a shoddy set of "tyres" (the antenna)? Furthermore, although I am in total disagreement with statements claiming "achievable ranges" one must not discount the ranges obtainable using seriously designed radiators, or worse still, the lack of range when a shoddy "RF sponge" is used. This latter statement has two consequences,
Is this cost worth the saving of a few pounds/dollars/euros/whatever?
2) Antenna design - Know the design, or if RF is not your strong point, have someone who knows antenna principles evaluate the design. The best person for the job (and this is a punt!) is a radio amateur. This crowd of nutters (I'm one of them) know designs that are efficient and work as opposed to those that don't. The best type of 'ham' is one who has built his own antennas, especially at the frequencies you use.
It is also a matter of importance to know the supplier has such confidence in his work that he even invites you to come and see his factory and how your antennas are being built. There are so many designs on the Internet that anyone could build an antenna that works, it's just whether or not you could do this cost effectively.
3) Inherently good designs - this is a tricky subject but the following has never let me down. Unless the antenna is an inherently balanced design such as a centre fed half wavelength dipole the need for a good groundplane is essential. Please note, do not believe those who state you do not need a groundplane for something like a five-eighths whip etc. There is one design, although good in principle, the end fed half wavelength dipole, but if not fed properly becomes tricky to match. There are many designs but all fall flat if not based on the J-Pole (please, do yourself a favour and believe me). The true J-Pole is just that, a J-construction, in other words, the bottom of the antenna is a DC short!
The variation to the J-Pole is the "Slim-Jim" and is a little more stable and not as susceptible to dielectric changes i.e. accepts being built in the open an then slid into a PVC tube. If any help is required on this please yell but do have a look on the Internet first, these designs are studied and printed ad-nausium.
4) Connector on the antenna - I cannot believe how such a simple thing is so difficult for most suppliers to do. This however is a personal preference. Having the connector on the bottom of the antenna allows for either RG58 or 213/4 to be used depending on the application and coax length. I must be honest, I find connectors mid-stream (lengthening a coax) a bother and potential faults.
5) Balanced Feed! - Yagis have one basic requirement, balanced feed! I have seen too many Yagis using the typical impedance transfer method with a piece of 125 ohm coax to get from 50 to 300 ohms and this terminated into the driven element. This forces the radiation pattern off the centre line and therefor pointing the Yagi in the required direction does not necessarily mean you are getting maximum signal. Again, consult your RF friend for advice.
Using these antennas choosing principles should ensure no come-backs and every installation working without fail (and with signal strengths never previously obtained!).
| | Ask a Question |03.11.00