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Author Topic: Poor SWR with 40m hamstick  (Read 1073 times)
AD5OS
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Posts: 23




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« on: January 04, 2005, 01:38:16 AM »

Hello

I have a question for all the elmers that run mobile hamsticks.

I have 3 sticks, a 20m Pro-Am, a 40m and 15m Hamstick.  The 20m does fine and the 15m does alright as far as getting a decent swr, but the 40m has a hard time tuning a lower SWR then 2.1:1 at its resonant frequency.  It does get a 1.3:1 when I use it with a magmount right next to where I have it installed on the trunk of my car.  I borrowed my buddies MFJ Antenna analyzer and it seems to me that the antenna is tunned right. I did notice that the capacatance increased with the magmount which might have helped it get a better SWR.  I thought perhaps I didnt have my car grounded well enough but checking with an ohm meter fromt he front to the back of the car is only 1 ohm resistance, every peice of metal seems to be grounded to everything else.

Does anyone have any suggestions to get this 40m to work any better?  Or of a better way to tune these up other then getting an antenna tuner or some type of device that I would have to mess with when changing sticks?

Thanks in advance.
Brian
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K0BG
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« Reply #1 on: January 04, 2005, 07:59:14 AM »

First off, unless you know how to to interpret the MFJ's reactance readout, you can't tell if it is plus or minus j. The main thing you should have been paying attention to is the resistive component at resonance! Even as lossy as the Hamsticks are, it is not uncommon to have the 40 meter input impedance in the neighborhood of 30 ohms, maybe a little less or a little more depending on how and where it is mounted. This represents an SWR of 1.6:1. A good quality antenna maybe as low as 20 ohms. This is why some sort of matching is almost always needed. You can check my web site under UNUN to see how to better match the antenna to 50 ohms.

Alan, KØBG
www.k0bg.com
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AA4PB
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« Reply #2 on: January 04, 2005, 08:00:19 AM »

The 40M Hamstick will have an impedance of about 25 ohms at resonance in a good installation which gives a 2:1 SWR. The mag mount probably just adds more loss and more loss raises the feed impedance. It looks better on the SWR meter but performs worse.

I've had good luck using a 2:1 UNUN (an unbalanced to unbalance broad band impedance transformer) with Hamsticks. In my installation it gave me a good SWR on all bands and doesn't require any changes to it when changing bands.

As I recall, Palomar has ready made 2:1 UNUNs.
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W7DJM
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« Reply #3 on: January 04, 2005, 08:13:51 AM »

To add to the above, you MUST remember the very basic premise that SWR and resonance have nothing at all to do with each other.

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KA5S
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Posts: 229




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« Reply #4 on: January 04, 2005, 11:54:06 AM »

Brian,

It is quite possible you need to ground your mag-mount to the vehicle chassis. A strap even as long as 8 inches or so will probably help.  I've had good results grounding a magnetic mount to the hardware on the hatchback and trunk lid hinge points.

Magnetic mounts do not have enough capacitance to the vehicle body to provide a good RF return path at 40 and 80, though at higher frequencies this is less of a problem.

Cortland
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WB2WIK
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« Reply #5 on: January 04, 2005, 11:54:30 AM »

That's true (about SWR and resonance).  Also true is that SWR, resonance and performance all have nothing to do with each other.

Another truth: Can't rely on DC Ohmmeter measurements to determine if you have a good ground or not.  RF current travels a different path from DC and RF resistance (impedance) is always higher.  I've found an Ohmmeter to be of about as much value in assessing mobile antenna installations as a string tied around a brick.  (The string/brick combination doesn't help, either.)

WB2WIK/6
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WB6BYU
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« Reply #6 on: January 04, 2005, 01:43:55 PM »

Sounds like your antenna has an impedance of 20 to 25
ohms at resonance, and you just have to match that
impedance to 50 ohms.  In addition to the 2 : 1 step-down
transformer, you can add either a coil or capacitor
across the feedpoint (and retune the antenna) to do this.

The formulas are in the ARRL Antenna Book, but I don't
have a copy handy.  Personally I'd try a 330pf "postage
stamp" mica capacitor from the antenna to ground, then
lengthen the whip for minimum SWR.  If that isn't close
enough, try a different value for the cap and try again.
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K0BG
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« Reply #7 on: January 04, 2005, 03:22:48 PM »

Let's correct a myth. There are many ways to match two diverse impedances. In this case, 25 or so ohms to 50 ohms.

When it comes to mobile antennas, it is always best to use a transmission line transformer (UNUN), or a shunt coil (actually an LC with the C coming from the antenna). The reason is simple. These methods provide a DC path to ground which aids in static build up and minimizes any problems cause by the antenna hitting an overhead power line. Using a CL (where the L is from the antenna) does not provide a DC path and should be avoided.

Using an UNUN and an LC (as suggested above) is redundant.

Lastly, UNUNs can be made switchable and if built correctly, impedances from about 12 ohms to 50 ohms can be easily matched.

Alan, KØBG
www.k0bg.com
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AD5OS
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Posts: 23




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« Reply #8 on: January 05, 2005, 02:13:28 AM »

Thank you Gentlemen.

All of your suggestions and advice are extremely valuable to me.  Alan, KØBG  your site is fantastic!  I wish I had found it before I started all this I don’t think I would have had so many questions about what I am doing.  It would seem that I still have a lot more bonding to do. It should have occurred to me that a voltmeter would not be an effective at the HF spectrum.  I have been using 12 gauge multi-stranded wire and #8 1/2" self tapping screws for my bonding but I didn’t think to bond the tail pipe or the engine block cause I just figured they would already be bonded.  I will work on bonding those this week sometime.

Ok now here are some more detailed readings from my antenna analyzer.  

on 40m with the whip all the way out the resonant frequency was
6.742  R=8 X=0 SWR=5.2
with the what as far in as possible I had it resonant at
7.00 R=6 x=0 SWR=6.0
7.16 was the lowest SWR I could get with R=55 and X=48
Of course I can cut the whip to get a shorter length to have it resonate at higher frequencies.


on 20m after I got it tuned up there is how the antenna analyzer looked at it.

14.000 R=14 X=3.6  SWR=3.6
14.100 R=11 X=2    SWR=4.2
14.200 R=8  X=0    SWR=5.0
14.300 R=7  X=0    SWR=
14.350 R=7  X=2    SWR=6.0

On 15m
with whip all the way in I had
20.800 R=75  X=0  SWR=1.5
21.000 R=70  X=0  SWR=1.3
21.300 R=67  X=0  SWR=1.2
Darn didn’t write down the value for the top of the band sorry.


ok I am seeing a pattern here...
When the X=0  the antenna is "resonant"  correct? Because I can get the antenna to get a lower SWR when there is a combination of R and X, but I don’t want that. I want to cancel out the X and just see a pure resistance.  I got this right, right?  Now someone mentioned that I should see my hamstick resonant with a 20-25ohm impedance... but it seems that it is much lower, around 6-11ohms on 40m and 20m and around 70ohms on 15m. Does this sound plausible or could my analyzer be mis-calibrated?

So now what would be the best way to match the antenna.  The UNUN or an automatic tuner?

Ok so if I make an UNUN to match the antenna like the one K0BG has on his site...  I am looking at making an UNUN that is 5:1 and have taps of for various ratios.  BTW… if I use a 2:1 UNUN it would match an antenna at 25ohm or 100ohms correct? I mean these are not directional correct?

Or could I just get an automatic tuner like the LDG Z-100 to do this for me?  The LDG says it will handle up to a 10:1 match. And because I am using a Yaesu FT-100D this can utilize my tune button on my rig. Will this tuner basically act like an automatic UNUN?

I am really just looking for some friendly confirmation in my assumptions, making sure I am not totally screwing this up or have the totally wrong idea like I did about the bonding or how to determine when the antenna was resonant.

-Brian  AD5OS
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AD5X
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Posts: 1426




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« Reply #9 on: January 05, 2005, 06:56:48 AM »

I know that Alan doesn't agree, but I prefer the base matching capacitor method.  Very simple and gives you a perfect match.  I use a switch box as part of the antenna base (February 2004 QST), and I can get a perfect match with any antenna by switching in the correct capacitor.  All the mobile rigs I've used recently (SG-2020, IC-706G, IC-703) do have DC continuity to ground so static build-up hasn't been a problem.  You can add a high value resistor across the capacitor if this worries you though.  

Phil - AD5X
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AA4PB
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« Reply #10 on: January 05, 2005, 07:01:17 PM »

I question the readings you are seeing on your analyzer. An impedance of only 6 ohms at resonance on 40M doesn't sound like the Hamsticks I've worked with. The feed impedance includes the radiation resistance plus the loss in the loading coil plus the loss in the ground system. The radiation resistance should be somewhere around 2 ohms so that would put the efficiency of your 40M Hamstick at about 33% - a really good Hamstick.

My experience has been that the feed impedance is more like 25 ohms which would put the efficiency at 8% which is closer to what I would expect.

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N4ZOU
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« Reply #11 on: January 06, 2005, 10:01:41 AM »

A shunt inductor for matching the vertical to vehicle ground is the best way to do it. You can homebrew your own coil at little cost and use the same coil on other bands as well. Never use a switch for selecting different tap points on an inductor or capacitors used for a matching network in a mobile matching setup. This causes a drop in efficiency and vibration of the whip element and vehicle body will shake it to peaces. Use an alligator clip to establish the tap points and then solder a male terminal to the coil point to be used and a stranded flexible wire with a female terminal soldered to the end. This provides the less loss for a moveable connection. The best way is soldered connections but this limits you to a single band and you would need a shunt inductor  for each band mounted to each whip element.
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WB6BYU
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« Reply #12 on: January 06, 2005, 12:16:43 PM »

Perhaps my earlier post was mis-interpreted:  I didn't
mean to use an LC match in addition to a matching
transformer, rather that they were additional methods
that could be used.

The shunt capacitor may have a slight advantage in
radiation, as it puts more current onto the lower
portion of the antenna mast.  But the difference is small
enough that any of the three methods would be suitable
for fixing this particular problem: use whatever is the
most convenient.

I suspect that you are taking your measurements through
the coax cable:  if so, you can't rely on the actual
values of X and R to tell you what is happening at the
antenna feedpoint unless you correct for the length of
the coax between the antenna and the tuner.  The
resonant frequency probably is very close to the point of
minimum SWR, and that will give you some idea whether to
shorten or lengthen the whip.

However, presuming you have a reasonably good ground
system, it is a reasonable guess that the antenna
impedance is below 50 ohms rather than higher, so if
you take the minimum SWR (resonance) and divide 50 ohms by it,
that should give you a good estimate of the actual
antenna impedance.  So if the SWR was 4 : 1 at 6.8 MHz,
for example, it would indicate that the antenna was
tuned too low in frequency, and probably had an imput
impedance of 12 to 13 ohms.  Use this value to calculate
the required matching components (using whatever method
you like) and then adjust the whip for minimum SWR
at your favorite frequency and see how close you got.

Yes, this method does make some simplifying assumptions
about the behavior of reactance and resistance, but it
should get you close enough to be usable, and from there
you adjust the values for a better match if desired.

Instead of a switch, you can mount banana jacks at the
base of the antenna and plug in an appropriate matching
method for each band when you change whips.
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VE3WMB
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Posts: 282




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« Reply #13 on: January 07, 2005, 04:44:17 PM »

As N4ZOU suggested, a Shunt inductor may be the easiest way to get a better match.  This is effectively a helical hairpin match.  Lakeview (the Hamstick people) make such a beast and is is called the WD4BUM inductimatch.  In simplest terms it is an inductor across the feedpoint.  Physically it is a 3 inch air wound coil that mounts at the base of the hamstick with a ring terminal and slightly overlaps the lower part of the antenna. There is a short lead with a an aligator clip that is used to tap the coil to ground, effectively putting inductance across the feedpoint.  For more info on why this works look up hairpin match or helical hairpin match in the ARRL Handbook or Antenna book.

I would also suggest that you consider adding a ground strap to your magnet mount as well.

Best of luck

Michael VE3WMB

P.S. Contrary to popular belief the characteristic impedance of a short loaded vertical is not 50 ohms. The shorter and more heavily loaded it is the lower the impedance.
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K0BG
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« Reply #14 on: January 09, 2005, 06:41:22 AM »

Brian, I'd take this off-line but you didn't leave your e-mail address.

The UNUN on my site is a 4:1 which is easy to build and there are plans in the ARRL handbook as well as on my site. The taps on the secondary provide the odd impedances. If you do it correctly, the maximum losses will be 2 or 3% even when using it as an auto-transformer (1:1 match).

As pointed out, capacitive matching does indeed work, and in some respects it is easier to do. However, it does not provide a DC ground AT THE ANTENNA! This has very little to do with any DC ground the radio has internally. And if your antenna is as long as mine is, if you did hit an overhead power line, well you get the point.

If you have addtional questions, you may e-mail me anytime.

Alan, KØBG
www.k0bg.com
k0bg@aol.com
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