Discussion in 'QRP Corner' started by KK4NSF, Dec 9, 2015.
There are better and best antennas for QRP if antenna losses are considered.
You might want to view this thread on portable antennas too:
I saw that thread.
As you yourself said, a favorite may not be the best.
Actual antenna tuner loss is a variable result depending on a wide variety of external factors including mismatch, load, transmission line used, frequency of operation. The type and quality of tuners internal components used in the construction of the tuner can also be a factor. When using an antenna tuner you may encounter loss occurring in the transmission line and not necessarily in the antenna tuner itself.
For example, if your antenna impedance is between 25 and 100 ohms (SWR of 2:1 or less with 50 ohm cable) then the loss encountered will be minimal however, if the tuner is required to work near it's limits, losses will be higher. Most loss occurs in resistive components such as the inductor coil.
It's a blanket statement because the actual numbers you are looking for are a moving target.
There is also loss from that LED SWR bridge on some tuners, that is switched out on at least two popular QRP tuners.
My comment was for the OP as they might have missed it. You don't miss anything on here.
Understood. That poses the question: in the cases where the impedence mismatch is rather large, wouldn't an impedence transformer like a balun be more effective? An end-fed antenna typically has a much higher impedence than a center-fed (maybe 300 ohms vs 50 ohms).... so a 6:1 ratio transformer would bring it into a reasonable tuning range with lower losses. That might explain why the MFJ 971 works with end-fed wires.... it has an internal transformer / balun.
Yes it does have an internal balun and this helps to align impedance in range of the capabilities of the tuner.
Half wave end feds are typically in the range of 1800 to 5000 ohms. It is entirely possible to ditch the antenna tuner and design an impedance transformer to bring the impedance in line with the transceiver, but then it would become a mono band antenna and you would lose it's multi band capabilities. Typically in order for an antenna tuner to accommodate the end fed, it will usually consist of an L network with the inductor in series and a capacitor will be installed on the antenna side shunted to ground. This will have some advantages including voltage protection and provides increased bandwidth.
Another example where impedance mismatch is large is when a 20m antenna is used on 10m where the impedance can reach several thousand ohms. This is a classic example where feeding the antenna with coax would exhibit large and unacceptable losses.
So to answer your question, no inserting an impedance transformer in the case of a rather large mismatch instead of using the antenna tuner is not going to be particularly effective. In my example above, you are much better off to feed the 20m antenna with 300-450 ohm feedline instead and use the 4:1 balun inside the antenna tuner which will convert the impedance to 75 ohms resulting in a 1.5:1 SWR. This is a much more acceptable loss than using coax to feed the same antenna when operating it on the same band. It also maintains the idea of using the 20m antenna as a multiband antenna arrangement.
I have to assume you are talking about a one man show for field day?
First choice would be a flat top 80 meter dipole fed with parallel feeders. Now you will have all bands from 80 to 10 meters. Since that is rather large and you may not want to use parallel feeders, I feel the best choice is an Off Center Fed 40 meter antenna fed with coax.
That is my favorite antenna whenever I set up portable. Model number ocf40qm. If space permits I do use an ocf80q since it also covers 12, 17 and 80 meters. We use this model for our field day at the DLARC site.
With the ocf40qm I never use a tuner. It covers 2, 6, 10, 20 and 40 meters. When portable it’s up only 20 feet and it works great.