DSFO25-ATV

The DSFO25-ATV is a high performance K1FO yagi antenna design cut for the 422 thru 439 MHz ATV portion of the band. It is similar to the DSFO432-25 except that it is tuned slightly higher in frequency to optimize performance for ATV. The design was slightly modified in November, 2005 to raise the maximum frequency up to 440 MHz for better match at the high end. Should you wish to update an older version made before November, 2005, contact the factory for details. Actual return loss curves for the DSFO25-ATV are reproduced at the bottom of this page. You can readily see the wide bandwidth there!

The idea was to design a yagi that delivered great performance at 70 cm. ATV frequencies, but in a size that did not require boom supports. This 25 element yagi utilizes a 17' 3" boom with rather large diameter tubing, 1 3/8" in the center, and tapering to 1 1/4" on each end. The final product is very rugged and will stand up to any ice storm or high winds you may encounter. We have a photo of a 4 yagi DSFO432-25 array with 4 1/2" of ice covering it. The boom hardly sagged! The DSFO25-ATV is supplied with a T match feed and type N connector 50 ohm input connector. The mast clamping arrangement utilizes a 3" x 5" extruded plate and is attached with stainless steel u-bolts.. Mast size may be up to 2' diameter. This design has been carefully checked out in the real world to ensure peak performance.. This antenna provides 16.5 dBd maximum gain up to about 440 MHz, and will provide great performance between about 422 to 440 MHz. The K1FO yagis possess quite a good bandwidth characteristic, and this trait produces a design that is remarkably stable in wet or dry weather. Forward gain, wide bandwidth, and exceptionally clean patterns are the most notable characteristics of this antenna, as with all of the K1FO style designs. The net result is wide beamwidth, and a clean lobe structure coupled with a very high gain figure. Clean H-plane patterns translate to efficient stacking with maximum possible gain and manageable array sidelobes, should you wish to stack these antennas in a larger array.

All Directive Systems antennas are verified for proper performance on an antenna range with state of the art swept measurement equipment to record the results. Some performance curves have been provided below. You can also download the instruction manual in PDF form.

 

DSFO432-25 YAGI ARRAY photo by WW1M

To evaluate the bandwidth, click here. To look at return loss (VSWR), click here.

Note that the graph depicts return loss rather than VSWR. As a reminder, remember that -14 dB return loss is equal to a 1.5:1 VSWR and -9.5 dB return loss is 2:1. Anything below -20 dB signifies an ultra flat match. Your wattmeter will act like it is not connected when viewing such reflected power levels. You can adjust the T match to favor a particular frequency if you wish. These curves are the factory settings as published in our instruction manual.

The DSFO25-ATV specifications are listed here. PDF Format (Instruction Manual)

The DSFO25-ATV yagi employs a traditional modified T match and half wave balun to match your 50 ohm feedline to the driven element. The T match and half wave balun provides a great method for achieving proper match along with exemplary decoupling of the feedline from unwanted antenna currents. At these higher frequencies, gamma matches and other matching methods used on lower frequencies, start to behave erratically. They tend to unbalance the feedpoint and cannot provide an effective rf ground as a result. The result is poor decoupling of the feedline and the antenna boom itself! Poor decoupling effects can reduce forward gain by large amounts. If any power flows on the outside of the coaxial cable, or along the antenna boom, gain or efficiency suffers drastically. Poor decoupling will show up as varying VSWR as the feedline is handled. Deep response nulls within the passband of the yagi are a tip off as well. Any skewing of the main lobe off the boresite heading indicates out of control rf currents caused by poor (or no) decoupling.

A sample close up of a typical Directive Systems "T" match is shown here. The actual antenna shown is a shorter rear mount 11 element 432 yagi (DSFO432-11R) driven element, but it is quite similar in appearance to the DSFO25-ATV.

SAMPLE DRIVEN ELEMENT T MATCH DETAIL

The DSFO25-ATV computer modeled patterns are here:

E plane and H plane.

ATV activity is currently spread across a major portion of the 420-450 MHz amateur band. Generally, the activity is clustered either near 420 MHz or up near 440 MHz. The DSFO25-ATV has been optimized for best performance at 430-440 MHz, and very good performance down to about 422 MHz. It is possible to fravor the high end or the lower frequencies near 422 MHz by a simple adjustment of the T match setting. If you are working ATV below 424 MHz exclusively. it would be a good idea to use the companion antenna, the DSFO432-25, which has better performance at 421 MHz, and NOT the DSFO25-ATV. We have reproduced the bandpass curves for both models so that you can make an intelligent choice depending on your requirements.

DSFO25-ATV T- Match tuning notes

Since ATV operates over a relatively wide slice of the spectrum, tuning for acceptable VSWR is more important than setting an antenna for one spot frequency like 432.100. Other problems, such as ghosting can appear as a result of VSWR as well. It is possible to move the frequency of best match on any of these antennas by simply re tuning the T match dimensions for best SWR at your operating frequency. We are including a few examples to demonstrate this with the DSFO25-ATV. Below is the standard T match setting VSWR (actually return loss) for the high end ATV portion of the band near 439.25 MHz.

FACTORY SETTING #1

These settings have the T match bars spaced at about 5.5" between the inside edges, and a T match wire length of 9.75". Note that the SWR is very good at 439.25 (-28 dB return loss), while it is about -10 dB or slightly better than 2:1 at 424 MHz. By a simple retuning of the T match bars, sliding them out to the end of the T match wires at about 9" between bars, you will get the following curve. Note that the low frequency end is improved. The gain improves very slightly by a few tenths of a dB, and is reduced by a similar amount near 439 MHz. A better result may be had at lower frequencies by leaving the T match wires about 3/4" longer on each side.

This particular curve is set to show the 1.5:1 bandwidth. It is about 15.56MHz wide. The match at 424 MHz is now about 1.5:1. It is still better than 1.5:1 up at 440 MHz. Comparing the two curves, you can see how the curves have shifted downward in frequency with the different settings. Each horizontal division is 2.5 MHz, while the vertical divisions are 10 dB of return loss.

There are two settings listed in the owner's manual. Setting #2, which favors the lower part of the ATV band, produces a curve as follows. It is made with a longer T match wire, 5 1/8" vs. 4 7/8" for setting #1

FACTORY SETTING #2

Note that the 1.5:1 bandwidth is approximately 18 MHz with this setting, 422.9 to 440.9 MHz! T Match wires are trimmed at 5.125" long on each side of the feed, while the T match bar setting is 6.75" . VSWR is acceptable at 439.25. -16.4 dB return loss equates to a VSWR of about 1.4:1

Now all of these different T match settings do not change the gain of the antenna much at all. There is some slight improvement on either end when the feed is optimized for that end, but the difference is slight, a few tenths of a dB and not noticeble at all. Most of the gain change is a result of increased losses in the feedline when a higher VSWR is present on the feedline.

If you want to read more about adjusting your T match, check here. One thing you should be aware of, after reading these T match notes, is that all of these great VSWR patterns will be ruined by poor quality coaxial cable feedline. Much braided coaxial cable has intrinsic VSWR in the cable itself. This will show up in your shack as noticeable "bumps" in the antenna response when viewed through a long run of coaxial cable. It is always better to measure your antenna with your test set right at the antenna. Adjust the antenna for best VSWR, then, when things look good, connect your feedline and leave the antenna alone. Poor quality coax cable can cause the antenna to appear to have poor bandwidth. If in doubt, always check your coax cable for VSWR. You may be surprised!