Vic K2VCO wrote: This is not correct. As you suggest, placing the balun at the input of an unbalanced tuner is an attempt to solve the problem posed by operating a balun in the output of the tuner, where the impedance is uncontrolled. However, W7EL (I can't find the reference just now) has shown that this method is actually no better than the balun in the output!
----------------------------------------- That one keeps coming up, Vic. I've read W7EL's dissertation many times, and I've built tuners with baluns at the input that overcame the problems with one at the input. W7EL's argument seems to confirm mathematically that a balun looking into a highly-reactive load will perform equally poorly at the input or at the output of an ATU. That argument seems to ignore that, in actual practice, we adjust the ATU so the balun is looking into a 50 ohm load with very low reactance when placed at the input. Kevin Schmidt, W9CF, has a nice dissertation that includes Roy's findings and a reference to the original paper and where you can download it (http://tinyurl.com/ctkwb). Kevin agrees with Roy, but Kevin says that the reason it's "useless" to put the balun at the input is that it makes both "sides" of the ATU "hot". Of course it does! You need a purpose-built ATU. Others claim that not building a fully-balanced ATU with identical reactances (coils and capacitors) in each leg unbalances the antenna. I have not found that to be the case at all. After all, the idea is to use efficient coils and capacitors, so there is very little loss in them. The coils and capacitors become 'transparent' to the RF when the system is resonated. All that's needed is to make sure that stray capacitance to ground is kept to a minimum. That's true when routing the feed line as well. I've used unbalanced T and L matching networks at the output of a balun with excellent balance in the feeders. In my experience, there's nothing wrong with putting a balun at the input or the output of a tuner. There's a lot can go wrong if one subjects that balun to highly-reactive loads! Many of the popular tuners that have been sold over the past several decades were intended to be used with *resonant* antennas - antennas that presented a low SWR to the rig without a tuner. The popularity of "no tune" rigs that began in the 1970's created that demand. Sure, you didn't have to tune the output of your rig any longer but you had to have an antenna that presented, at worst, a 1.5:1 or 2:1 SWR to the rig. A lot of antennas didn't meet this criteria, showing a somewhat higher SWR to the rig. That's where the MFJ and other popular, medium-cost tuners came in. They could be put in the feedline and provide the required low SWR to the rig. Instead of tuning our final amplifier output circuits in our rigs like we had done since the 1920's, we now adjusted an antenna tuner to accomplish the same thing. Why not simply go back to tuned final amplifier outputs on rigs? The new Federal Communications Commission regulations here in the USA raised the standards for harmonic and other spurious signal suppression above what a simple tunable final amplifier output circuit could provide. More exotic multi-section filters were needed, like those in the Elecraft rigs. This is especially true above QRP levels. They aren't easily tunable. So, for years now Hams either connected an antenna directly to their rig or, if the SWR tended to range to high on some frequencies, used an ATU in the line to provide the low SWR the rig needed to operate properly. Those ATU's often included a balun at the output for hams feeding *resonant balanced antennas* such as a folded dipole. A folded dipole can be made up of 300 ohm TV 'twinlead' or similar line. It has a feedpoint impedance of 300 ohms. Using an ATU designed to look into something in the general vicinity of 50 ohms on the antenna side with such 300 ohm feeders was easy: use a 4:1 balun. The ATU would see 50 ohms, thereabouts. Other antenna configurations that presented a reasonably low SWR to the balun also worked well. Enter the WARC bands. Suddenly the old center fed wire using open wire feedlines that popular in the 1930's is back to accommodate all those bands! Forget feedline SWR, the ATU will fix it. Trying to use such an antennas with a modern ATU using a balun at the output led to a lot of grief and confusion. On some bands the system works. On other bands baluns simply convert most of the RF to heat. As others pointed out here, trying to measure ATU efficiency when feeding odd loads is a challenge. (The most straightforward way to do that is to put the ATU in an insulated box, measure the temperature rise while it's being operated and then calculate the amount of power converted to heat. Straightforward, but not especially easy to do). So now there's renewed interest in balanced tuners that can handle not only a wide range of impedances but can efficiently couple to balanced lines. A balun at the output can work FB on some bands, but it's tough to predict. It's even tougher to evaluate how well it's working since a few milliwatts radiated power can gain a 599 report halfway around the world under the right conditions. Personally, I use a standard link-coupled fully-balanced manually-tuned ATU to feed my doublet. It's not pushbutton quick. It sometimes takes me 10 or 15 seconds to retune on a new band. But it's very efficient and has a tremendous matching range across the HF bands. It's the same circuit shown in many old manuals and on Cebik's web pages http://tinyurl.com/7ez6r There's a picture of it above my K2 under my call listing on QRZ.COM. Besides, it looks "cool" to me (speaking of fashions <G>) and I can lay an NE51 under one end of the coil and watch it blink on and off as I send. (Tiny minds enjoy small amusements). Ron AC7AC _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
