Archive for the ‘Antenna Tuners?’ Category

Antenna Tuners? Transmitted signal efficiency

Saturday, September 22nd, 2007

This is a topic I just had to start up. I am a purist when it comes to Ham radio. Efficiency is what I worry about. I know I can’t get the ideal circumstances all the time and have to deal with poor efficiency some times. However, antenna tuners bother the heck out of me.

What is an “antenna tuner”?
Well, most people take the name literally and think it is something that tunes your antenna. Wrong.

An antenna tuner is not an “antenna tuner”. In Ham radio, all (or very close to all) of our radio systems have an impedance of 50 ohms. Antennas must present a 50 ohm load (or very close to that) to properly work. Now, that’s not to say a dummy load will work well as an antenna – it doesn’t. Hence the name “dummy” load. It presents a 50 ohm impedance to the radio to make it happy so it puts out full power, the “dummy” part means it doesn’t radiate (or very little) so it is safe to use as a test device for transmitting and not sending out signals on the airwaves. So, then to get a 50 ohm match at the radio through an antenna system that does NOT show a 50 ohm match we can do a little black magic – IE, use an antenna “tuner”. An “Antenna Tuner” changes the impedance of the transmission line or antenna (if a long wire) to 50 ohms before the radio. This makes the radio happy and able to transmit without worry into a good match. HOWEVER… and its a BIG HOWEVER – the match AFTER THE TUNER is STILL BAD!!! You did not change the impedance of the system, you just MADE THE RADIO HAPPY! Keep reading.

So, for an antenna to work well it must present close to a 50 ohm load to the radio. As the impedance changes so does your SWR (or Standing Wave Ratio). The higher the SWR the lower the power your transmitter will put out (in modern, solid state rigs) because the SWR-protection in the rig kicks in and drops the power so you don’t blow your finals.

Lots of times, Hams use wire antennas. Long wires, dipoles, doublettes, windoms, G5RV’s, etc. Hams in a pinch sometimes use one antenna on all bands. Well, this is where we get in to the efficiency issues. To get the most out of your antenna you need to have the antenna resonant on the frequency of operation and you need it to present a 50 ohm match to the transmission line (coaxial cable from rig to antenna) so the radio is happy. When you load up an antenna on bands it was not designed for you get impedance mismatches.

If you do not present a 50 ohm match, or close to it, to a 50 ohm impedance transmission line you LOOSE power (both RX and TX) out of the transmission line. Coax loss is rated in dB (decibels) per 100′ per frequency of operation AT NOMINAL IMPEDANCE (50 ohms). Power, in dB, goes like this – every double in power is 3dB. If you show 50 watts to a 3dB loss system you get 25 watts out, likewise 500 out with 1000 in.

As the frequency increases, so does the loss. At HF we really don’t need to worry about the loss THAT much. That is.. until we show poor impedance to the line. A poor impedance drastically increases the loss in the line.

SWR, or Standing Wave Ratio, is a measure of reflected power v.s. input power. SWR works like this – any double in impedance is an SWR of 2:1 (2 times nominal). At the same time, any half impedance is ALSO a 2:1 SWR (nominal/2). For example: an impedance, in a 50 ohm nominal impedance coaxial feed line, of 100 ohms gives an SWR of 2:1. An impedance of 25 ohms to the same feed line also gives an SWR of 2:1.

A 2:1 SWR is the maximum that is typically accepted by modern radios. Even then, the SWR protection has already kicked in and dropped your power. Youll be lucky to get 60-70 watts out of your 100 watt radio with a 2:1 SWR.

Back to the cable itself now. Lets say the cable we are using is rated at 1.3dB/100′ loss @ 30MHz (LMR-240, a typical cable of the RG-8x size used by the majority of us). Lets also say we are running on 10 meters (30MHz is the high point of 10 meters), it is 100′ long, and we are presenting a 2:1 SWR to it. I haven’t done the exact calculation here (there are more variables involved with calculating this than just SWR and loss) but lets say that the loss increases to right around 3dB. That doesn’t seem too unreasonable, given our initial loss of 1.3dB/100′.

Now, remember – a 3dB loss is equal to 1/2 you input power. If you get 50% of your power through the transmission line then that means, with that 60-70 watts in (after SWR protection in the rig), your antenna is only seeing 30-35 watts out. Now, add in the efficiency of the antenna you have up. If you have a beam antenna or an antenna with some gain, then you can ADD power back in (gain means more of the radiated energy is concentrated in a certain direction). If you’re like most of us and running a 1/4w vertical or a dipole, then, unless it is optimized (and at a 2:1 SWR it isnt) you are LOOSING even more. 30-35 watts (most likely less than that) out of your 100 watt radio is probably not what you want, is it?

Now, I am going to bring in a whole other issue. Not only are you loosing efficiency (power, rx signal strength) through a bad match in coax you are ALSO generating interference, or RFI. Your signal loss through the cable when you transmit has to go somewhere. It is dissipated as heat, some of it radiates (IE, your coax is part of the antenna), and SOME of it is distorted and radiates as RFI trash. The higher the voltage at the feed point of the antenna and the worse the impedance match the more RFI you generate.

A dipole for 80 meters is roughly 134′. If you look at a graph of the voltage/current distribution on a dipole at its resonant frequency you have high current and low voltage present at the feed point. Voltage is opposite of current. Now, a dipole is 1/2wl – a “classic” dipole anyway. Lets use this antenna on 40 meters. 40 meters is half of 80 meters, which means the frequency is half as big. That means our 1/2wl antenna on 80 is now 1 full wl on 40, 1/2wl each leg. Now we need to change the graph of our current/voltage distribution. The voltage is still high and current low at the ends. However, the feed point of the antenna ALSO now shows high voltage/low current. In fact, the voltage is as high as it could possibly be at the feed point.

As I said before, the higher the voltage the more RFI generated. Higher voltage can also arc across longer distances. The capacitors in your manual tuner have plates that are close together. The higher the voltage through these capacitors the easier it is to arc across the plates. Arcing causes RFI and also severe damage to the capacitors. Every arc/spark of any voltage emits RF. The higher the voltage the more RF, and the more damage done – the more melting power the arc has. Unless you are building a spark-gap transmitter, arcs are BAD.

Through all of this I am explaining a desk-top, manual, in-shack, type antenna tuner mounted close to the radio with coaxial transmission line. There are ways to get around these poor impedances and losses, however! Keep reading.

Antenna tuners need not be close to the radio. They are convenient to have close to the radio so you can crank the knobs or push the buttons. However, if you can change the impedance of the antenna system to 50 ohms then why not mount the tuner close to the ANTENNA instead? This would give you a PERFECT 50 ohm, flat 1:1 SWR, impedance through the entire length of transmission line. This would minimize losses to the least possible. The only losses you have now are the tuner losses and the antenna losses (given its not optimized).

The only solution that makes sense here is an automatic tuner. SGC makes some good ones. The Icom AH-4 is popular, although poor matching range. Also, LDG makes a remote tuner, or you can modify a Z100 or similar for remote use. If you are operating on the low bands and want a tuner that can be remotely operated to tune the entirety of one band (160 for example) then you can make a loading coil with a lead-screw drive that will compress and expand the coil to adjust the impedance. Its pretty rough, but if you can build one for $30 youll save much more on the cost of an auto tuner.

Using a tuner on an antenna that is not optimized for the frequency of operation is not ALL that bad. In reality, all the power you are capable of sending to the antenna is getting to it. Gooch’s paradox: “RF gotta go somewhere”. If you get your RF to the antenna it will radiate. Its that simple. The performance of the antenna (gain, pattern, etc) might be an issue… but it will radiate.

Another way of getting around the impedance problem with a tuner is to use balanced transmission line instead of coaxial cable. For this you need a balun. Most manual tuners with a balun have a 4:1 balun. However, it might be beneficial to use a 1:1 instead. If you have a long antenna and tune it up on a high band you have a low impedance. For this example Ill use 40 ohms. If you use a 4:1 ballun at the tuner the impedance the tuner actually sees is now 10 ohms (40/4=10). 10 ohms is harder, MUCH harder, to match than 40 ohms is. In fact, 40 ohms wouldn’t even need a tuner.

The problem with balanced line is how you route it. It can not be close to anything conductive, or relatively conductive. That includes tree branches and leaves (the water, sugars, saps all have some conductivity and will throw off the balance of the line). Maintain at least several inches (the more the better) of space between the line and anything else. This causes problems getting the line in to the shack. Going through a window with a spacer to hold the line at a good distance from the frame is a good idea. Going through a wall is OK as long as the line isn’t close to anything metal in the wall (nails in studs, wires, ect).

Balanced line, when used properly, is the BEST transmission line to use. 450 ohm is the most common, others include 300 and 600 (true “ladder line”). I’m sure there are other values as well.

Remember, an “antenna tuner” doesn’t actually tune your antenna. It merely is an impedance matcher so that your radio sees a 50 ohm impedance. There are many other factors you must consider in order to have an efficient, RFI-free system.

If “antenna tuners” hurt your efficiency and just aren’t that great to use, then why do I have one? There are lots of situations when tuners are great to have. They are a must if you want to use a dipole and cover an entire band (edge to edge). Ill use 40 meters for example. You can get a 2:1 SWR width of one section or the other – SSB from 7.150-7.300 and CW/digital from 7.000 to 7.150. but to cover a whole band a tuner is the only way to do it. To minimize losses in both ends you tune the antenna to 7.150, right in the middle, and use the tuner to get the edges. Or, if you operate CW more then tune the antenna for CW and use the tuner for SSB, or the other way around. Longwire antennas need a tuner. A longwire can be used on any band, given it is 1/4wl at the lowest frequency you intend to operate on (the longer the better). Without a tuner, a longwire would be useless. Balanced line antennas (doubletts for example – a random length “T” antenna fed with ladder line – NOT a dipole) need a tuner unless the antenna is optimized for one frequency. Basically, any antenna that is not resonant at any specific frequency needs a tuner.

Even I use non-resonant antennas. We all have to at some point. If I had my own place with 50 acres to put up radio towers and antennas then maybe I wouldn’t need a tuner. Until then, I need to use compromise antennas. I understand how my antenna systems work and I must accept the efficiency I can get. However, I CAN make my systems as efficient as I possibly can (short coax, balanced line instead of coax, close impedance, etc). The name of the game is to minimize losses. That’s all.