Wednesday, January 8, 2014

Simple Ham Radio Antennas: When to use antenna tuners. Post #253.

Nowadays, the antenna transmatch ("tuner") is an integral part of many amateur radio stations.  With this device, a ham can match the output of his/her transceiver with the often unknown impedance of the antenna itself.  With a suitable balun (balanced to unbalanced) transformer, it is possible to use one antenna to cover several HF bands, especially if you're facing space restrictions on a small lot or have  to operate in "stealth" mode because of HOAs and CC&Rs.  This situation applies mostly to the use of 300 ohm tv twin lead and 450 ohm ladder line, which require a balun (1:1 or 4:1) and antenna transmatch to operate as feed lines in a multiband, single antenna system.

There are times when you may not have an antenna "tuner" at your disposal, especially if you're operating in portable or emergency situations.  In my case, I remove my Drake MN-4 and MFJ 941-E transmatches from the antenna system for routine maintenance several times a year and must make do with no transceiver matching devices.  Are there times when you can still operate without a "tuner" and not endanger your transceiver with high SWR and possible damage?

The answer is a qualified "yes."  I faced this problem over the New Year's holiday when I took my Drake MN-4 "tuner" off line for maintenance and was looking for ways to work my limited amateur radio schedule while fixing the old Drake.  I ran across an article by Steve Ford (WB8IMY) in the January 1994 issue of "QST", pp. 70-72.

Ford says the first step is to determine if you need an antenna "tuner" in the first place.  He says a "tuner" is needed if you find yourself in these situations:

1.  Your antenna is fed with open-wire.  A suitable balun and antenna transmatch is needed to form a "bridge" between balanced feed lines and the unbalanced output (usually 50 ohms) from your transceiver.

2.  When you use your antenna on bands other than the frequency range you prefer.

3.  Your antenna has a narrow SWR bandwidth.

Ford says a "tuner" may not be necessary if the following conditions exist in your station:

1.  If your SWR is less than 1.5 to 1 or less on the frequencies you use.  This is most noticeable on 40 meters, where, if you cut the antenna for the lower part of the band (below 7.050 KHz), you can operate on the SSB portion of 15 meters.  In this case, the antenna will work on the third harmonic of 40 meters.  A similar situation can work for an 80 meter/10 meter combination.  The assumption here is that you're using a good grade of 50 ohm coaxial cable for your feed line.

2.  You have high SWR at VHF and UHF frequencies.  Although "tuners" for these bands exist (i.e. several from MFJ), Ford recommends you "correct the mismatch of the antenna by adjusting whatever type of mechanism it provides."  You can also check for cable defects and make sure the cables have been installed correctly.

3.  You're interfering with TV, telephones, and electronic entertainment devices in your neighborhood.  Although antenna "tuners" may reduce harmonic radiation, some RF problems will remain.  These problems might be caused by fundamental overload and the poor design of entertainment devices.  Many electronic devices don't contain any rf filtering devices at all.  Some interference may be picked up indirectly by cables and speaker leads.  About all you can do in these cases is reposition your antenna, reduce power, and use digital modes such as cw and PSK-31.  You may have to shift your operating hours to avoid interfering with your neighbors.  

So, there are times when an antenna "tuner" may not be needed.  If this is the case, what kind of antennas can we use to "fill the gap" until the antenna transmatch is repaired?

I've found two multiband antenna designs that don't require antenna "tuners".  I've built both antennas and found that they work very well with SWRs below 2:1 on several amateur radio bands.  That will be the subject of the next few posts.  So, stay with us.

REFERENCES:

http://www.arrl.org/file/Technology/tis/info/pdf/9401070.pdf.  (originally published in the January 1994 issue of "QST", pp. 70-72.

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Aloha de Russ (KH6JRM).

BK29jx15--along the beautiful Hamakua Coast of Hawaii Island.
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