Since my Antenna Basics page was introduced the amount of antenna related mail I have received has reduced considerably. But it is quite obvious that there are a lot of misconceptions and misunderstandings. Here are a few selected questions I have received by e-mail regarding antennas, together with my reply.
Is it not logical to assume "The more metal you have in the air, the better the antenna? Why not just use a longer wire?"
It is quite logical to assume that, but it may not be 100% correct. If all the metal you have in the air is resonant or active, such as a Yagi antenna, tower or mast then yes I agree. To simply increase the length of a radiating element without regard to theory will only reduce the frequency of resonance to another frequency you may not be interested in. Go to my Antenna Basics page and read about resonance, then come back here on.
The best transmitting antenna is also the best receiver antenna!
Ah! That old chestnut! Yes, and no. Transmitters and receivers have very different properties. The best transmitter antenna is one that radiates all the power you shove up there. It must also radiate it in the right direction and be sited so that other nearby objects (including the ground) do not absorb the power. Let us talk for a moment about volts. A transmitter will give out a signal of typically 100 volts (into 50 ohms). We want as much of this signal to be received at the other end as possible.
A receiver antenna, on the other hand should ideally catch as much of the signal as is available, but the signal voltage is often as low as 0.000001 volt, frequently smaller. At these sort of signal levels there are, unfortunately, loads of other signals we do not want, such as solar noise, atmospheric noise, and a huge variety of man-made noise. The best transmitting antenna will, by definition, receive everything.
If on the other hand I had an HF loop antenna made from coaxial cable and earthed the braid, I would receive nothing. Break the braid half-way round the loop and it will receive again. The wanted signals may be only 30% of the level using an unscreened loop, but static and other such signals will be eliminated. The effects of assymetric capacity to nearby objects will also be eliminated. You could get an interference reduction of 80%. In this example your signal has gone down to 30% but the interference has reduced to 20%. Clearly the difference between the wanted signal and the other rubbish has improved.
Another difference could be the use of loop or frame antennas using their directional properties to "notch out" on-frequency signals coming from other directions. Again there is a large improvement in received inteligibility, but not necessarily an improvement in signal strength.
I have recently moved house and now live on a small hill. I have a mast on top of the hill, beside the house. Excellent for transmitting, but the dipole in the trees at the bottom of the garden gives me a weaker but clearer signal than the transmitting antenna besides the house. One of the reasons is domestic interference. In my house I have computers, TV sets, video recorders, washing machine, tumble drier, central heating, flourescent lamps, ... the list goes on. All of these generate crap that can be received with HF antennas. Siteing the antenna in a "less than optimum" position almost eliminates all this rubbish for me.
To take a different view, the difference in signal voltage level between a transmitter and a receiver is typically 100,000,000:1 which is difficult to comprehend (like comparing one mile with the distance to the Sun) so it doesn't take much of a disturbance to overpower such a microscopically small signal.
If received signals are weak to begin with, then you could be interested in getting the biggest signal possible from the antenna and try to do something with the signal at the receiver end. "Throwing money at it" usually gives good results, as any DX contest participant will tell you. For us mere mortals, it is more economical to get better antennas than buy a better Yaesu rig with more knobs, dials and those nice chrome "pushy-things" on the front.
My basic antenna problem is the poor SWR reading. I can never get it to 1:1, but the best I can get is usually about 1.3:1 so is there a fault with my SWR meter? Am I doing something wrong?
Yes, you are doing something wrong. You are looking at the numbers too closely. When you put a shelf on the wall your wife says she wants it 80cm from the floor, would you be bothered it it were 80.128375cm or only 79.998125cm? That is EXACTLY what you are doing with VSWR measurements. Let us take a look at the power fed to the antenna system and the percentage reflected back as a VSWR:
| Fwd | Ref | VSWR | Efficiency |
|---|---|---|---|
| 100w | 50w | 5.7:1 | 50% |
| 100w | 25w | 3:1 | 75% |
| 100w | 10w | 1.9:1 | 90% |
| 100w | 5w | 1.6:1 | 95% |
| 100w | 2w | 1.3:1 | 98% |
| 100w | 1w | 1.2:1 | 99% |
| 100w | 0.5w | 1.15:1 | 99.5% |
If you are getting a VSWR of 1.3:1 then 98% of the power sent to the antenna system is not coming back. If you wanted to get that 2% more then take your VSWR meter out of circuits as the losses of the instrument are far greater than the 2% you are loosing. You are probably loosing at least 10% of your power in the feeder cable, but even that is less than 1dB. The guy at the other end will never know the difference, even if you were to switch the loss in and out whilst he scrutinises the S-meter with an intence scrut. A VSWR of 2:1 or better is almost perfect.
Harry, I want to tell you about the best commercial antenna that has ever been sold. It only cost me £175 ($260) and the VSWR is perfect from 1.8MHz to 30MHz. The antenna is only 5-metres tall and does not need a groundpla... blah, blah ... sell you one if you like?
I have never ever in my whole life spent so much money on an antenna and I doubt if I ever will do. I know the antenna you have recommended to me, and NO, I am not interested in buying one from you.
Try transmitting 100 watts into it for a few minutes then go have a feel at the matching box. It will be hot! This is because most of your TX power is being burned up in a dummy load. There is a sort of directional coupler using a ferrite device and this is used to direct the reflected power to the dummy load. In fact, 95% of your power is being burned up in the load resistor and the 5% being radiated by the simple whip antenna only gives a 3 S-point reduction at the other end. The very good VSWR figure you are measuring is due to the dummy load impedance in the matching unit, because that it about the only device you are metering.
This is the sort of thing that happens when people buy ready made equipment without any knowledge of what is inside it. I have seen a fake burglar alarm in a car that was nothing more than a flashing LED in a plastic box. It sells for $25 (250:- här i sverige) but the total component cost is only about $2, including the bit of wire and the LED. Your antenna is probably worth about $20 plus any cables that are supplied. Many hams would be happy with it because the VSWR figure is so good.
The next time you buy anything, ask the salesman if you can have a look at "the gubbins inside". Hamradio shops may sometimes oblige, as will all car sales showrooms. It does not work when you buy a washing machine, though!
Best regards from Harry - SM0VPO
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