Notes On An Optimum Portable HF Antenna, a Comparison ofMFJ927 and SGC239 AutoTuners, and a 31 ft. vs 43 ft. vertical at VE1ZAC

 

JeffVE1ZAC††† July 2012

 

Thatís a mouthful of a title, but conveys what I want to talk about regarding my vertical antenna activities this spring. This all started from last years Maritime QSO party roving operation which entailed setting up an awkward trap dipole at three different sites in SW Nova Scotia. The antenna worked fine but it was a laborious and time consuming effort to erect a center push up mast, put lines in trees, etc. An effort was made over the winter to study other types of antennas that would be equal or more effective and much easier to put up and take down.

 

Choosing the contest antenna:

A list of goals looks like this:

-Light weight, and easily put up by one person if needed

-Car top installation or on ground if needed.

-80,40,20,15 and 10M coverage needed, with 6M if possible.

-Complete control of antenna matching from car operating position to allow rapid band changing.

-100 W CW max power

-20,15,10 and 6M should have low take off angle (10 to 20 deg)

-40M should have medium take off angle (20 to 50 deg.)

-80M should have medium to high take off angle (50 to 90 deg)

 

Well, that list is easy to fill with a vertical ! But working a typical 9 foot mobile vertical with a loading device is very inefficient on 40 and 20, the two work horse bands for these QSO parties. Assuming you can make a good match, antenna radiation is almost entirely determined by the current carrying part of an antenna. Loading devices donít radiate, they just provide reactance and impedance transformation to make a good match.

 

I am a long time owner of one of those nifty German made carbon fiber push up poles that are 30 feet long. They weigh only 2 lbs and are very reliable. That would make a nice 30 foot vertical with a lightweight wire taped to it. And support of the pole base is not very difficult. So, what would a 30 foot vertical look like with some elevated radials ? Turns out.. pretty darned good. It has all the features needed in the short list and reasonable gain on every band to boot. It is a little short for 80M but it doesnít have to be a great performer there anyway. 80M is for some very short hops to pick up some close in multipliers. An EZNEC model with the base about 5 feet off the ground (on top of the car) and at least 6 20 foot radials looks pretty good on all the bands and great on 40 and 20M. So this looks like the way to go.

 

Matching is a bit of an issue. A 30 foot vertical is fine for direct coax feed and the rig tuner on 40M and 15M, but the feed point is very reactive on 80 and 20M. These bands will require some help.

 

The following table shows a comparison of measured and EZNEC calculated feedpoint impedances of my backyard30 foot vertical with radials on all of the HF bands. We are only interested in the non WARC bands for the contest of course, but for future reference, they WARC bands are included. I wonít go into how I arrived at the stub length here, but it is designed to be helpful for an elevated vertical with sloping radials, and is 41 feet of RG6 with a far end short. I tried three different length stubs, and this one turned out to be the most useful.

 

SWR

Rohms

X ohms

Z Magnitudeohms

Mhz

Bare

c/w Stub

EZNEC

Bare

c/w Stub

EZNEC

Bare

c/w Stub

EZNEC

Bare

c/w Stub

EZNEC

 

 

 

 

 

 

 

 

 

 

 

 

1.81

80

3

>100

76

62

1.1

-544

64

-835

550

90

835

3.56

43

6.5

>101

30

215

4.6

-248

-152

-375

249

263

375

7.06

1.6

2.9

4.1

51

23

26

25

-26

-50

57

35

56

10.12

8.5

5.2

7.6

375

25

98

135

61

160

399

66

188

14.06

10

3

18.5

147

37

860

-224

-49

233

268

62

891

18.78

3

2.5

21.5

84

109

108

-66

40

-322

107

116

340

21.06

2.3

2.5

13.4

116

83

46.5

7.5

-52

-163

116

98

170

24.9

4.5

1.8

2.2

173

27

69

-93

0

44

196

27

82

28.06

4.2

2.2

8.6

117

92

269

-99

-37

206

153

99

339

* Measurements with AIM4170C at†††††††† antenna feed point

** Stub is 41 feet of RG6 with short at far end.

 

Note that 160M, 30M, 17M and 12M are not used in the contest. Further, 10M was of low interest. So, 80,40, 20 and 15 are the prime bands, with 40 and 20M being the work horse bands. As you can see, the matching improved on 80 and 20 with the stub, as expected, and was slightly worse on 40 and 15..but not enough to create a big problem.

 

Note the EZNEC impedance calculations werenít that good compared to measured numbers. That is mostly because of the ground vagaries. I wasnít really worried about that, but included the models so I could get an idea of relative far field strengths and take off angles while ignoring the match. I did spend some time building a calibrated vertical model to match this real antenna, but that is a subject for another article.( Note to self.. write about this, lots of interest)

 

The original goal was to use a remote auto tuner at the base of the vertical, with the stub in place, to optimize the match for each band. The backup plan was to leave the tuner in bypass and just use the internal tuner of the K3. This would work on all bands except 80M.

 

In a separate article on the MCC 2012 QSO Party on this website, I described the mobile setup used in the contest. I wonít repeat that information here, but itís worth re-hashing the failure of the MFJ 927 remote tuner during the first hour of the contest while setting up the mobile antenna at the first site. ( Likely self induced failure, BTW) That forced our team to rely on the backup plan of using the K3 rig tuner and leave the remote tuner bypassed throughout the contest. Results with this antenna were acceptable and satisfactory, as predicted,but it would have been very nice to have that original remote tuner working.

 

Antenna Choice Conclusion: A portable 30 foot vertical antenna, with 6 to 9  of 20 foot radials and a base mounted remote matcher/tuner is a very useful portable/mobile HF antenna. Considering the much improved setup time and effort, it is a winning choice. The stub is worth the effort to lower the stress on the matching device whether at the antenna base or inside the transceiver for 80 and 20M. The full 30 foot element on 40M proved very useful in the contest as conditions local werenít that great late in the evening. However DX from Eu was rolling in and many DX QSO party contacts were put in the log thanks to this vertical.

 

So, onto the next part of the story.. namely a comment on two remote tuners.

The Remote Auto Tuners

 

I originally picked an MFJ 927. Good price, sturdy looking case and good connection choices.I wound up returning the first one for a variety of reasons, and obtained a warranty replacement before the contest. This unit was tested in the yard with the mobile antenna successfully before the contest, yet failed during the contest setup. Very frustrating. I will deal with what went wrong with the unit, but after the contest, I ordered an SGC 239 to evaluate in the same application. It is my intent to use this antenna setup again for mobile operations, and want a reliable base mounted auto tuner. The SGC 239 performed flawlessly in my backyard tests. The two units are not exactly the same, so I have done a detailed comparison, and will comment on both units.

 

Here is a summary comparison of my two units:

 

Side by Side comparison of MFJ927 and SGC239

5/7/2012†††† Check means "Winner"

Item

MFJ927

SGC239

Note

MFJ927

SGC239

Outdoor case

1

3

0

General Mechanical

3

5

Connections

2

5

3

Bypass

3

5

2

Coax Power bias

5

2

Impedance Range

2

5

Board Quality

4

2

5

Manual

5

2

5

Service

6

4

3

Warranty

5

5

Price

5

5

Operating test

7

3

5

Reliability

8

1

4

Buy again ?

2

4

Totals:

47

53

Point Scoring system:
(Highly Subjective)

1 to 5

Very Good

5

N/A

0

* Notes:†† 1

MFJ has decent external casewith plastic and SS, but gets very hot in sunlight

2

MFJ has coax connectors and binding posts

3

SGC needs power for bypass operation

4

MFJ board very rough soldering

5

SGC fabulous manual and technical downloads

6

MFJ very good at supplying parts for DIY repairs, online trouble ticket system excellent

7

MFJ failed twice during tests and min use.

8

Made DIY improvements to MFJ, but SGC worked perfectly during same tests.

 

Point by point comments;

 

Outdoor case:The MFJ unit comes in a sturdy plastic weather shield with a beefy SS base. The base is nicely made but I found the coax connector clearance too short and had to use coax elbows to make connections. OK, but not wonderful. The plastic box is black and gets very hot in the sunlight. I had a Tupperware container with a flip lid at the antenna base to house the tuner and stub, keep connections dry, etc. I found the lid had to be left open and a sun shield put over the unit.†† The SGC unit, on the other hand is not weather proof at all and requires installation inside a container of some sort for outdoor use. Here is a picture of the antenna base unit with a modified MFJ 927 tuner package.


The cover has been removed from the modified 927 package. (More on modifications later in the story). The stub is plugged into the coax connector which is paralleled with the red binding post antenna wire connector. The common mode choke is on the input feed line from the transceiver.

General Mechanical: The MFJ 927 is mechanically well made. The SGC 239 is lighter and requires installation in a weather proof container. No real contest here, MFJ is a good mechanical package. Wish the cover was white, though.

 

Connections: MFJ 927 has very good set of connectors underneath the SS frame. My only complaint was the height allowance for a PL259 connector and coax is too tight, necessitating use of coax angles. A small complaint, really. The SGC 239 has terminals for everything. I added a simple set of connectorsto match the MFJ 927 setup. One good thing is the interface control connections on the SGC 239 and external push buttons, if you want to use them. You have to dismantle the MFJ 927 to get at itís control buttons. One important point.. the MFJ uses a common earth side coax connection for input and output, the SGC does not. Pay attention to this. Thatís the reason for the (red) plastic piece I added to hold the output connectors. The SGC does not have a built in bias T like the MFJ, so I added one which serves as the input connector. The SGC also has a nice set of external indicator LEDs which aid in diagnosis.

 

This provides a similar set of connections to the MFJ unit, making it easy to change the units during tests, or for operating failures during a contest.

The banana plugs on both units are for easy attachment and experimenting with radials.

 

Bypass mode: The MFJ is bypassed with no power. The SGC requires power on to be bypassed. In operation, this is no problem, but when something goes wrong, I prefer the MFJ system which bypasses in power down mode. If the SGC gets funky on you, you have to turn it off and then on again for bypass, and there really is no easy way to tell it to stay bypassed from then on. So, MFJ has a better system, I think. I plan on adding two SPDT relays at the SGC 239 that will put the unit offline when there is no power, and switch the tuner inline when power is applied. Subject for another little article.

 

Coax Bias T: I do some weak signal, MW DXing and understand the perils of inducing noise via power over coax schemes. However, for ham contesting, I really do not think this is an issue. For that reason, I find the compromise of pushing DC out to a remote tuner over the coax an acceptable solution. The MFJ has the necessaries for this built into the tuner, and the shipmen comes with an MFJ bias T for the transceiver end of the coax. The SGC has no facilities for this. You either run a power wire to the unit or add the bias T units, as I did.

 

Impedance range: The MFJ is 2 to 1400 ohms and the SGC 239 is .2 to 5000 ohms. TheSGC has a broader range and in my test vertical setup, never had a struggle with a match on any frequency, stub or no stub in place. The MFJ unit struggled with some of the more difficult spots, especially for 80 meter use of the 30 foot vertical without the stub.

 

Board Quality: The MFJ unit had an alarming pattern of long wire lead protrusions on solder side of the board that were all bent over at right angles. Itís a miracle there were no shorts on this unit. A half hour spent straightening and properly trimming the component leads lessened my anxiety on shorts. Soldering of components was fine otherwise, but several wire jumpers on this until were hack jobs. The SGC had no issues and looked very professional all the way.

 

Manual: The MFJ manual is in the package and downloadable from the website. It is adequate for the task, ( barely) although it takes a few reads to get the feel of where it is going. The 927 is the same board as used in several other units with slightly different programming for the default ďremoteĒ features. Less clear is the fact that you can open the unit up and use the two interior push buttons and power switch to make this thing look like the other similar MFJ shack mounted products. It is also very easy to knock this unit off its default settings. A very nice feature is the built in diagnosis routines which can be access when the unit is opened. These are explained in the manual, but it takes a while to understand whatís going on.

 

The SGC manual is the ďGold StandardĒ of tuner documentation! It comes as a 500 Kb PDF document that not only explains fully the use of the tuner, but is a document that should be used in any radio amateur course to explain how matching works ! Really excellent stuff. The SGC website is a little cryptic, but itís worth a visit to download this or other manuals or their free PDF technical documents on antenna theory and matching. Very well done. Kudos.

 

 

Service and Warranty: Untested for SGC so I have no comment. MFJ has pretty good service to my mind, if you know how to tap into it. They make extensive use of a web based trouble ticket system which works very well. I have made use of this system many times and have never been disappointed. MFJ is also very generous in sending you parts if you want to try and fix your own problem, especially if you know exactly what you want. They have never charged me for failed push buttons, fried board components, even a blown PIC for the 927 ! This is one of the nicer aspects of dealing with MFJ, especially if you are a good technical person yourself. The warranty claims I have made for MFJ products were done through dealers, and always came to a happy conclusion. I have bought several MFJ products from DX Engineering who gave exemplary service on warranty claims with prompt shipping. MFJ also readily provided some testing and service information not provided in the manual, through their trouble ticket system, which resulted in a personal email with PDF documents attached. I really appreciate this kind of service.

 

Price: In Canada, the SGC 239 unit is less expensive than the MFJ 927, but it does not include some connectors and the coax bias T system, which kind of evens things out. If you include those items, and the fancier case of the MFJ 927, itís probably a slightly better deal for the MFJ unit.

 

Operating Tests: Best to list these, in order.

 

1)    Previous year, fall 2011, 1st MFJ 927 tested with yard 30 foot vertical, at end of 150 foot coax run. Unit failed after 1st hour of tests and was returned for warranty. In these tests, power was limited in all cases to 5 watts.

2)   Replacement MFJ 927 tested with yard vertical in spring of 2012 and on mobile antenna setup in yard for testing before contest. 3 hour test session, no problems.

3)   During setup in QSO party with same mobile vertical antenna setup, MFJ 927 failed. Later testing found failed pic, several relay driver transistors and the 25 ohm precision ceramic R used in the impedance measuring circuit. This was likely self induced ( by me) because of an error in power delivered during the tuning function from a K3 transceiverÖ the lesson here is to be exceedingly careful about sending a low power Ďtuneí signal to these remote antenna tuners. They are easily damaged.

4)   After contest, MFJ until was repaired by me and some changes were made to the unit to make it more use full in this application, by my reckoning. More on this later. The unit tested out reasonable well on the re-setup mobile antenna in my yard during over night tests. It doesnít repeat as well as it did when it was new, leading me to believe it still has some calibration or functional issues, but it does work. 5 watts for tuning and 100 watts for on air tests.

5)   SGC 239 unit was purchased and adapters added. It has been operated in over night tests on my yard vertical and on the temporary mobile setup. Flawless operation. Low power for tuning (5 watts) and 100 watts for on air tests.

6)   Both units were extensively bench tested into a dummy load and a reactive OCF dipole antenna, with the unit mounted at the shack position.

 

Reliability: OK, this is a tough one. I am a little nervous about my known reliability of the MFJ 927 unit, and I donít have enough operating experience with the SGC 239 yet. So frankly, they are probably evenly weighted at this point.

 

BUY AGAIN Decision:Well, I would definitely buy the SGC 239 again, for this application as it worked flawlessly during the tests. My only caveat is the bypass mode, which would require physically removing it from the circuit if it fails.

On the other hand, if my modified MFJ 927 stands the test of time, it may prove to be the superior unit for remote antenna use. Frankly, when it comes down to it, both have some minor details that need addressing before use, but I think the real decision would be based on this bypass business and how important it was to me. Right now, if it was critical, the MFJ 927 gets the nod, and if it isnít, then the SGC 239 is the one. When I add the bypass relay to the SGC 239, the choice may tip in it's favour.

 

So, next contest, I will test both units and take both to the contest, but probably start with the SGC 239. With my connector setup it takes only a few minutes to change them out.


WARNING, from the cheap seats :If you canít insure that tuning is consistently done at low power during the heat of a contest, you should probably avoid using these things at all ! I do not believe either unit is built well enough to do tuning at power over , say 50 watts for any length of time.

 

Here is a picture of my favoured mobile antenna setup:

 

The radials all plug into the base with banana plugs and are help out with bean bag weights. The antenna pole is pushed up with the base on the ground and then the whole thing is lifted to the car racks and held in place with big ty wraps. It takes about 15 minutes to setup or take down. This looks like the way to go for any repeats of a contest or operating event needing a mobile or portable antenna. No trees required. BTW, it was tested on 6M with no auto tuner at the base, and it worked fine with the rigs tuner.

 

Some ideas on the MFJ 927

 

Above is a picture of the layout in the MFJ 927 with the plastic cover removed. Below is the same unit after my repackaging and repair efforts.

 

After two failures of this device, I spent some time studying the failures and other small inconveniences and decided to modify my unit as well as repair it. Here are the things I changed:

1.    New aluminum case, to isolate PIC controller from high RF fields

2.   Case painted white to stay cooler in bright sun.

3.   Case now totally enclosed for RF shielding

4.   Replace low wattage Caddock ceramic resistor with 20 watts of Ohmite ĎPulse Eaterí resistors.

5.   Add more binding posts to facilitate radial hookup and experimentation.

6.   Modify control LEDs to be visible from outside of case.

7.   Repair unit from previous damage.

 

The case is a standard aluminum Hammond chassis with a cover plate. The little bulkhead allows further (not perfect) shielding for the PIC control board. The LEDs were removed from the control board and mounted in a block of plexiglass attached to the side of the chassis. You can now see these things from outside while troubleshooting or monitoring operation. Very handy.

 

 

Damage found on the boards from tuning with too much power included blowing the PIC controller, taking out two relay driver transistors and destroying the ceramic 25 ohm resistor used in the impedance measuring bridge. This resistor is designed to be mounted on a heatsink, and I suspect it would have survived if there had been a small heat sink on this unit. It is mounted vertically on the chassis and it would be easy to add a small clip on heat sink to this deviceÖ something I would do to any new MFJ 927.

 

In my case, I decided to add a higher power version of this resistor for further experimenting. In a compromise, this also added a bit of reactance to the R leg of this circuit.. which may not be a good thing. For the time being I will leave it as is.

The extracontrol board shielding may not be strictly necessary, but canít hurt either. One problem I have not been able to fix is the SWR bridge calibration using the built in software routine. My unit does not seem to want to easily engage the dummy load while in the calibration stage of this routine, which requires a little further troubleshooting, and the PIC doesnít seem to behave in the way expected in the instructions for this procedure. The PIC does exhibit every other function correctly, however.

 

In the end, I have a functioning unit, but it is not very repeatable. Itís probably just me however. I plan on giving it one more set of ďFactory defaultĒ instructions and trying it with the outside vertical antenna again. It looks like it is very close to being useable.

 

I found two relay surface mount driver transistors cooked. I replaced these with regular NPN switching transistors with leads that are soldered to the original pads. A not too difficult repair.

 

 

The 30 foot vs 43 foot vertical antenna choice

 

A little sidebar arose during all this vertical antenna and matching work.

There is lots of talkabout how wonderful a 43 foot vertical is, compared to other vertical setups. I have a 30 foot vertical with a base mounted switched match setup which allows it to work very well on 80,40,30 and 15M.Not much goodness to report on 20M, although it will load with help from the rig tuner, leaving a high SWR on the coax feed line. Itís not a great trick for me to add 13 feet to the top of this antenna, but I figured I better look at what that might get me before going to the effort.BTW, there is a very good analysis of 43 foot antennas at http://www.hamradio.me/antennas/answer-to-everything-43-feet-antenna.html

andhttp://www.hamradio.me/interests/43-foot/††† His conclusions seem to match mine.

 

The problem with the 30 foot vertical is 80 and 20M. I donít worry much about the high bands as I have a 2 element Steppir yagi available which performs very well 20M and up. But, itís always nice to have a selection of antennas to utilize for DX hunting and contests.

So, how great is the 43 foot antenna ? Well, it looks like the only real advantage is the lower reactance at the feedpoint, making matching an easier go. But do you actually gain anything ? EZNEC gives us a few good hints. Look at the following comparison sheet I made up with gains of the two verticals over medium to poor ground (thatís me !)

 

Side by Side comparison of 31 ft and 43 ft vertical

5/7/2012†††† Check means "Winner"

Item

31 feet

43 feet

Note

31 ft
@ 20 deg

43 ft
@ 20 deg

Gain at 160

*

0.25

0.23

Gain at 80

-0.43

-0.45

Gain at 40

-0.64

-0.47

Gain at 30

 

 

 

-0.41

0.19

Gain at 20

***

 

0.29

2.04

Gain at 17

1

2

0.65

Gain at 15

2

2.28

-0.51

Gain at 12

3

 

-0.29

1.18

Gain at 10

4

-0.74

-2.31

Totals

2.31

0.55

Score

Score

Matching needs

5

5

3

Installation ease

6

5

3

Totals:

 

12.31

6.55

:Hybrid

Point Scoring system:
(Highly Subjective)

1 to 5

Very Good

5

N/A

0

* Notes:†† 1

43 has gain of 4 dBi at 40 deg

2

43 has gain of 3.7 dBi at 40 deg

3

Both have 4.4 dBi at 40 deg

4

Both have over 4 dBi at 40 to 50 deg.

5

Auto tuner for 43, existing switched passive for 31

6

31 in place, 43 needs 13 foot extension, new guys.

*

Gains based on EZNEC model over poor ground

***

Even though 43 much better 20Mvertical, have a Steppir beam for 20M

 

The conclusion is that the antenna can give a .6 dB relative gain on 30M and a 1.7 dB relative gain on 20M. Consider that an S unit is around 6 dB, and you can see that this antenna is not going to change very much. It may be worth doing to get a better 20M signal leaving the antenna, but I already have a yagi for that band, so itís hardly worth doing.It looks to me like there is little to be gained by doing the work to add 13 feet, the same as there is little to be gained by adding an auto tuner at the base, compared to my existing ultra-reliable switched matching unit. (There is some description of this unit in other articles on this website, and a picture of the matching unit in the picture sidebar) For 15 and up, the 43 footer is actually quite poor for low angle work of any sort.. it wants to push most of the radiation out at high take off angles.

 

Considering the needs for increased guying and a little more handling work, itís not on my antenna upgrade list.

 

I hope this vertical information is useful to some of you folks making antenna decisions in the near future.

 

Jeff†† VE1ZAC