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Author Topic: 14 mhz frequency with low SWR for 17 meter whip. Help  (Read 3275 times)
KF7ZFC
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Posts: 121




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« on: October 22, 2017, 11:31:15 AM »

I am using the approach as outlined in this article:

For FlowerPot Antenna info see: http://www.zianet.com/dhassall/flowerpot.html, http://www.zianet.com/dhassall/flowerpot.pdf.

I am not using the switching network they used, because is not needed.

I have a similar counterpoise as was described. Mine is 13 foot by 4" wide aluminum flashing with the antenna mounted in the center and 2 ten foot wires at each end, Before running the SWR on the 17m whip, I added a 12 foot by 4" aluminum flashing section at right angle to the 12 foot section near the center.

But my lowest SWR for a new MFJ 17 meter whip is at 14.000mhz.
I had the same problem with both a MFJ 40 meter and 20 meter whips both brand new also reading it's lowest SWR much lower than the specified frequencies.

I have a brand new RigExpert AA-30. I called the distributor and they replaced it with a new one. The new gave the same results.

I tested the SWR with a 3' cable, 40' cable and the 100' cable which will give me access into my house. SWR/frequency did not make much of a difference with the cable length.

I live in Chandler AZ with dry clay like soil. The article looks like it was also in AZ.

I do not have a direct ground near the antenna yet and can't install one at my transceiver. The plan is to drive an 8 foot copper rod into the ground about 6 or so feet from the antenna. I will install a lighting/emp suppressor made by Industrial Communications Engineers which I have.

The article did not mention a ground or the kind of problem I have, so I am at a loss of what my problem is.

HELP!

Steve
KF7ZFC

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KF7ZFC
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Posts: 121




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« Reply #1 on: October 22, 2017, 02:03:33 PM »

Did further testing

Built a 18.1 mhz dipole (155" length) mounted only 3 feet off the ground. Used 3 foot cable on RigExpert AA-30 meter, lowest SWR was at 17.2 mhz

While I was out there I retested the 17m whip (again using 3 foot cable) and lowest SWR was at 13.9 mhz .

Starting to feel that the Rig Expert is faulty. This is a new one that the distributor sent.

Is there something else?
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KC4ZGP
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Posts: 1637




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« Reply #2 on: October 22, 2017, 03:20:15 PM »


I have an AA-30 as well. I use it to get me in the right direction and then I use my Bird
meter for the final touch.

Don't put too much stock in it. A quality ntenna analyzer costs a tad more than a couple
of hundred bucks. The one I use at the base is made by Agilent. It starts at 50 MHz
however. I would love to have one but I haven't got a few hundred thousand dollars.

Kraus
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N9AOP
Member

Posts: 680




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« Reply #3 on: October 23, 2017, 08:49:34 AM »

When camping sometimes I use one of those 17ft whips on an outpost tripod.  The 20M SWR is low across the whole band.  Of course the ground conductivity is very different in NE Illinois than in AZ.  What reading do you measure and what do you consider acceptable for operation?  Many radios will show SWR at the receiver and you can compare that to what you get from your device.  They should be quite close.  And no- you do not need a $1000+ device to measure HF frequencies.  The November QST compares the various rig expert analyzers.
Art
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KF7ZFC
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Posts: 121




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« Reply #4 on: October 23, 2017, 09:00:05 AM »

It appears that I have another faulty AA-30 analyzer.

With a dipole cut for 18.1 mhz the AA-30 says its at 17.2 mhz.

It was suggested by the ARRL Lab that I  check with a receiver the AA-30 operating or indicating  frequency. It was  an operating frequency of 17.23 mhz

I am waiting for the dealer to tell me what the next step is
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KF7ZFC
Member

Posts: 121




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« Reply #5 on: October 23, 2017, 09:05:13 AM »

it appears that the AA-30 is operating on the wrong frequency and giving erroneous readings on top of that.

That is why I cut an 18.1 mhz dipole to measure. This would eliminate any potential for faulty Hamsticks.

The AA-30 measured 17.2 mhz for the 18.1 mhz dipole.
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WB6BYU
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Posts: 17195




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« Reply #6 on: October 23, 2017, 09:58:37 AM »

Being close to the ground will change the resonant frequency of the dipole,
and the standard formulas are only approximations anyway.  I'd be more
prone to assume that you got a 20m antenna by mistake rather than a
second bad analyzer.

Does the resonant frequency shift when you change the whip length?

Replace the top whip with a piece of #14 solid copper wire and trim it
down.  See if you can get the resonance up to 17m.  On the other hand,
if removing the top whip doesn't change the resonant frequency, then
the resonance is not the one you are looking for.

I had a 80m dipole with a dip around 5 MHz, and spent a lot of time trying
to shift it.  Turns out that was due to my coax length - when I looked further
there was another dip that the dipole length affected.
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KF7ZFC
Member

Posts: 121




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« Reply #7 on: October 23, 2017, 10:52:02 AM »

The 17 m whip section is fully extended. I previously lowered the whip and it did not change that much.

In 2 posts back I noted that the ARRL Lab suggested I measure the output of the AA-30 on a receiver.

I did and it measured 17.23 mhz. This was the frequency of lowest SWR on the 17m dipole that was cut to 18.1 mhz. The 17 m Hamstick measured even lower

Would you expect that much of a change in frequency close to the ground for the dipole? Zack at ARRL did not comment about that.

Remember that my original measurements on new 40 m and 20 m Hansticks also read very low

With the operating frequency of the AA-30 at 17.23 mhz and it reading low SWR, it appears that there is something wrong with the AA-30

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KF7ZFC
Member

Posts: 121




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« Reply #8 on: October 24, 2017, 01:02:14 PM »

In spite of the RigExpert AA-30 being a second brand new unit, the dealer finally said it to was bad and said send it back.

I am waiting to see if they refund my money or send me a third unit.

I did learn a lot about antennas and testing in the process.

Knowledge is a good thing
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AF6AU
Member

Posts: 64




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« Reply #9 on: October 25, 2017, 09:03:55 AM »

Handy but simple tools to have are carbon resistors that you tested with a GOOD ohmmeter, and soldered inside a pl259. I have a 50, 75, 100, and 150 ohms. These will give you a great handy reference to calibrate with. Carbon or metal film resistors are not inductive, installed inside a PL259 with short leads they are frequency flat from DC to well over 500Mhz. 100 ohms should get you a nice 2:1 reading, 50 ohms, a drop dead 1:1, 150 ohms, 3:1. I like 2 watt carbons, the fit nice, and will take a short brief transmission of 10 watts easy. You can test most rigs at low power too.

I have several in 50 ohms, and keep them around for testing HT's and my old mechanical VHF/UHF swr meter. Nice to see the actual accurate wattage (and if the battery is good), and that the old meter that gets banged around still is trustable.

The soil does weird things to antennas, remember a lot of whips are considered good if the SWR on a metal car body is 1.5:1 at lowest. Reality of that 1.5:1 can be 35 ohms. So we off tune the antenna away from resonance to get a reactive to add to that, as long as the transmitter output transistors are fooled into seeing a 50 ohm load they are happy.

Also many antennas are delibrately made long, so you HAVE to trim them. I bought a set of 5 foot hamsticks (80,40,20,15,10), and the 15 meter whip needed nearly 6 inches cut off before it tuned on 15 meters.

A prior gentleman mentioned to try to substitute the whip with another wire and begin cutting it down to bring up the frequency. I agree, if the whip wants shorter for higher frequency, clip another wire before the good one.

Arizona soil is dry, but it's also loaded with iron oxide (the red color). That may have an inductive effect. Iron in my red tint tile roof made tuning a 20 meter delta loop impossible, as the bottom of the triangle was parallel to the roof surface and 4 feet above for 22 feet. And if it rains (yes even Arizona), your dry tuning on iron loaded soil now becomes conductive capacitive soil, and the tune changes all over.

You can try to understand what is going on, but the goal is to get the transmitter happy, so try different radials, elevated radials, and swap a cheap wire for the whip tip, and fear not cutting it.



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WB6BYU
Member

Posts: 17195




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« Reply #10 on: October 26, 2017, 10:06:48 PM »

Quote from: KF7ZFC

The 17 m whip section is fully extended. I previously lowered the whip and it did not change that much.



If you moved the whip from maximum to minimum and didn't see much frequency shift on 20m,
then it doesn't sound like the 20m resonance has anything to do with the length of the whip.
Did you also check for a resonance around 17m?  Did the 20m resonance change if you
removed the top whip entirely?  I've certainly had cases where I've found resonances in my
antenna system that weren't affected by the wire length - most often they were due to the length
of the coax.

Also, the resonant frequency of such an antenna depends both on the antenna and the
ground system
.  Ground radials laying on the dirt are not resonant where you might expect
from the wire length.  So if you have a 17m antenna with a ground radial cut for 17m, the radial
likely will be effectively longer, and that will lower the resonant frequency.

If you replace the whip with a piece of copper wire that you aren't afraid to chop to pieces, you
can shorten it (without getting the bottom end inside the loading coil) and see if that will raise
the resonant frequency to where you want it.  At least it will give you an idea of how much the
whip changes the resonant frequency.


Quote

Would you expect that much of a change in frequency close to the ground for the dipole?



Well, let's see what EZNEC thinks...

I modeled a dipole 2 x 115 inches long.  At 40' in the air, the resonant frequency varies from
18.32 to 18.46 MHz depending on the wire diameter.  (That's an example that the standard
formula is only an estimate).  With #14 wire it is 18.4, which I'll use as the reference.
(Note that this is for bare wire - insulated wire will be resonant somewhat lower in frequency.)

At 10' the resonant frequency drops to 18.26 MHz.

At 5' it is back up to 18.38 MHz.

At 3' we're back up to 18.4, but then it starts dropping fairly quickly.

At 1' it has dropped to 17.9 MHz.

Varying the ground conditions changes the resonant frequency up to a couple hundred kHz.


But if I model the same dipole with a 16' feedline to the analyzer without a balun, I get two
SWR dips, at about 16.3 MHz and at 18.3 MHz.

How much difference would it make using insulated wire?  Conventional numbers are that it
would lower the resonant frequency by 3% or so (depending on the insulation type and thickness,
etc.)  That's over 500 kHz, which drops us down into the 17 MHz range.  The actual resonant
frequency of the dipole also depends on how it is built, such as how the ends are formed into loops
and any stray wire length between the end of the coax and the antenna itself.

So could the antenna really be resonant as low as you measured?  It is possible, depending on the
construction, but certainly down to 17.6 or so isn't unexpected, depending on the wire insulation.


One other test you could run is to hook up your transmitter and measure the SWR of your vertical
on 20m.  If it reads low (as you measure) then the analyzer may be giving the right result.

On the other hand, if it reads high, that doesn't mean the analyzer is giving you the wrong reading,
as the difference can be due to common mode current.  Plugging the coax into the analyzer (which
is small and isn't grounded) vs. plugging it into the transceiver (which likely is grounded through the
power supply, and certainly has more mass) will actually be measuring different antennas,
since the coax braid and everything attached to it becomes part of the antenna.



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KF7ZFC
Member

Posts: 121




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« Reply #11 on: October 27, 2017, 08:40:17 AM »

Thanks for your input.

To test the 17m hamstick I did set the analyzer originally at 18.1 mhz with a range of 3 Mhz. When I found no resonance I set the range larger and  larger and then found the resonance at 14 mhz.

The dealer took the analyzer back. I'm waiting to see if they give me a refund or send me a third unit.

If they refund my money I will probably buy: BX-240 : FA-VA4 Vector Antenna Analyzer kit $140. This appears to have good reviews  in eHam.

I paid $210 for the AA-30. I don't think there any other analyzers that are in this price range.
As a retired person my cash is limited.

Of course more testing once I have a new analyzer
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KF7ZFC
Member

Posts: 121




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« Reply #12 on: October 27, 2017, 02:27:16 PM »

It turns out that both AA-30's had the same problem, they are pulling a unit from stock and testing it to make sure this one is ok.

Gigaparts said that very few of RigExpert units have had problems.

They are also refunding the money it cost me for the 2 return shipments.

Gigaparts has been a very helpful through this. They are a good supplier

Steve
KF7ZFC
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AC2RY
Member

Posts: 310




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« Reply #13 on: October 27, 2017, 06:52:16 PM »

It turns out that both AA-30's had the same problem, they are pulling a unit from stock and testing it to make sure this one is ok.

Gigaparts said that very few of RigExpert units have had problems.

They are also refunding the money it cost me for the 2 return shipments.

Gigaparts has been a very helpful through this. They are a good supplier

Steve
KF7ZFC

I do not understand - did you check analyzer using resistors? If you didn't - you cannot say that they are both wrong. With your dipole I suspect the problem is your whole antenna system, which causes coax cable to be the radiator too. IF this is the case you may have measurements results all over the place and major change in SWR value if you simply turn dipole around 10-20 degrees.
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KF7ZFC
Member

Posts: 121




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« Reply #14 on: October 27, 2017, 08:47:54 PM »

The original antennas tested were new MFJ 7" Hamsticks 40m and 20m. The next antenna tested was ordered from another dealer was a 17m MFJ Hamstick.

All 3 antenna had "supposed" resonances considerably lower than they should have.

The dealer sent me a second AA-30 analyzer that read the same as the first,

I then  cut a 17m dipole to eliminate the the fact that the hamsticks were faulty. It to read low.

This second AA-30 unit was sent back to the dealer and they found it was also reading incorrectly.

They are shipping me a third unit after they insure it is operating correctly
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