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Author Topic: 40 m on 6BVT  (Read 933 times)
K4WEL
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Posts: 13




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« on: December 02, 2017, 01:36:02 PM »

I have good swr on 10, 15, 20, and 30 (less than 2.0) but cannot get below 5 swr on 40 on my Hustler 6 BVT?
 I have 26 thirty foot radials and buried RG-214.
Anyone have any recommendations?
Thanks in advance
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K0ZN
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Posts: 1774




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« Reply #1 on: December 02, 2017, 03:29:16 PM »

If the SWR is good on all bands higher in frequency than 40, then that tells you there is *something* wrong on the part of the antenna from the 30 M trap up. I mean this respectfully: have you double checked the measurements for the section(s) above 30....and make sure all connections are clean and good? An SWR of 5 says something is out of spec. in a significant way. I would expect that antenna to have a fairly narrow SWR bandwidth....what part of the band are you checking it in? Is there any large metal object less than 33 ft. away? Is the SWR lower an one end of the 40 M band than the other?

Are your radials bare copper wire or insulated?  I guess it is not impossible that if they are insulated and 30 ft. long, they could be causing a problem since 30 ft. is well short of a 1/4 wave. That is one of the reasons I like buried bare wire for radials, that generally eliminates resonance issues.

I have never owned that antenna, so can't be too specific about recommendations. Give a more details on the system and how the antenna is mounted, etc.

73,  K0ZN
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KC7YE
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Posts: 174




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« Reply #2 on: December 02, 2017, 04:33:42 PM »

Down load the 6BTV manual from DX Engineering, much more detailed then original issue. Make sure you are measuring correctly. Have had six BTV's over the last 50 years, all worked, all ground mounted with and without good counterpoise (much better with, HI HI) but all useable
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HAMHOCK75
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Posts: 396




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« Reply #3 on: December 02, 2017, 11:43:07 PM »

Not sure how unusual that is. I have a 5BTV. I first installed it without radials and the SWR's were almost exactly like the SWR curves depicted by NewTronics for it.

Then I started adding two radials at a time. Every time I added a pair of radials, the SWR was worse but only on 40M. By the time I reached 12 radials, the 40M SWR had degraded from 2.1 to 3.1 at 7 MHz.

Initially I thought something was out of adjustment but then I ran an EZNEC analysis of it on 40M. This behavior is normal.

Here is why. The 4, 5, 6BTV's are only about 17 feet long on 40M. In other words, it is more like 1/8 wavelength long on 40M. If it were 1/4 wavelength long on 40M the radiation resistance would be about 36 ohms which under perfect conditions would give an SWR of around 1.4.

I used EZNEC to calculate its radiation resistance at 7MHz with a perfect loading coil of the proper size to resonate at 7 MHz. The result was a radiation resistance of about 12 ohms. That would give an SWR of about 4.1 at 7 MHz rising to 5.8 at 7.3 MHz.

As I added resistance to the loading coil, the match got better and better again since the coil loss is in series with the radiation resistance. Below, is the EZNEC analysis with a perfect loading coil over the 40M band.



Funny but it is ground and coil losses that make the match good on 40M. The 5:1 SWR means you did a good job of getting those losses down. I stopped adding radials when the SWR became 3:1 because my tuner couldn't handle a SWR higher than that. Now have a new tuner that can handle higher SWR so might start adding radials again.
« Last Edit: December 02, 2017, 11:50:46 PM by HAMHOCK75 » Logged
K4WEL
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Posts: 13




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« Reply #4 on: December 03, 2017, 05:01:13 AM »

Thanks for the info, the radials are all insulated solid copper, I will try to add some more, when time allows, and see what happens.

Yesterday I went through the install as directed by DXE manual, all measurements were reset and each trap from 10m up came in with less than 2.0 swr until I got to the 40m section.

 There is no room left to shorten the 40m section unless I adjust the 30m trap down, just ran out of sunlight but will try again soon.  I will try a couple more radials then also.

Thanks for the replies
Don
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HAMHOCK75
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Posts: 396




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« Reply #5 on: December 03, 2017, 11:13:24 AM »

Quote
There is no room left to shorten the 40m section

The shorter the antenna is on 40M, the lower the radiation resistance, the higher the SWR.
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WB6BYU
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Posts: 17190




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« Reply #6 on: December 03, 2017, 12:35:55 PM »

With a good radial field (which you may already have, since you are having this problem) the 40m
tuning may reach the limit of adjustment.  This isn't due to radiation resistance, but rather the
interaction between the antenna and the ground.  (Think of a good ground system providing more
capacitance to the antenna, so it has to be shortened for resonance with the same inductance.)

I though the DX Engineering instructions for the antenna covered this situation:  seems to me
that it required shortening the lower sections and slightly readjusting those traps to restore
resonance on the lower bands, which then provides enough margin for the 40m adjustments.

But that's the first place I'd look for a solution.
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HAMHOCK75
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Posts: 396




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« Reply #7 on: December 03, 2017, 01:30:35 PM »

Additional capacitance does allow an antenna to be shorter and I would would agree that more capacitance either to ground or in the form of capacitance hats like the 5BTV has do not alter the radiation resistance. But someone could make big $ if they could make a monopole substantially shorter than a 1/4 wavelength with a radiation resistance near 50 ohms. I don't recall the formula but have seen the derivation for the radiation resistance for a monopole significantly shorter than 1/4 wavelength. There is a square in the equation so every time the height is reduced a factor of two, radiation resistance falls by four.

There is a simple test. Remove all the radials. The SWR should get better because ground losses will add to the low radiation resistance.

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K9MOV
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Posts: 85




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« Reply #8 on: December 03, 2017, 07:12:40 PM »

I have had a 6btv for many years. I ran into this problem years ago with my antenna. It ended up to be the 80 meter coil at the top of the antenna. It had opened up. I tried to get the antenna to work without it, but didn't have a long enough tube to get 40 meter resonance back. I got another 80 meter coil from a local and everything was back to normal on 40 meters.
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NK7Z
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Posts: 1846


WWW

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« Reply #9 on: December 03, 2017, 09:01:08 PM »

This antenna WILL work well on 40-10, see my blog called "Notes on tuning a 6BTV" located at:
https://nk7z.net/notes-on-tuning-a-6btv/
At the end of the tuneup article is a set of charts generated by my AA-54 RigExpert Analyzers.  As you can see, I have great SWR at the feedpoint, and in the shack.  My setup is 24, 33 foot radials, on the top of the ground, insulated wire.  Buried feedline, and no 1:1 balun.  Hope the blog entry helps you...  BTW: My soil is clay, with a water table 7 or 8 feet down.

If your 10-30 are working, then I would suspect the 80 meter loading coil as being open.  
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Thanks,
Dave
For reviews and setups see: http://www.nk7z.net
K4WEL
Member

Posts: 13




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« Reply #10 on: December 05, 2017, 06:32:43 AM »

Thanks for the info, I have not installed the 80m resonator as I don't plan to operate on 80. DXE told me that not installing the 80m coil would be fine and that should make no difference.
I will install the coil and see what happens.
Appreciate all replies and will try again when the rain stops.
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K4WEL
Member

Posts: 13




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« Reply #11 on: December 10, 2017, 05:22:51 AM »

Thanks to all for chiming in on this thread, I did attach the 80m resonator and when I powered up my TS-830s and my digital SWR meter on 40m the SWR was at 1.7 for the portion of the band I intend to use.  I will be fine tuning the entire antenna soon, at 28 deg F it is just to cold to play.

I was pleasantly surprised when I check 10m through 30m that all was well SWR wise, but they will be re-tweaked also.

I was initially informed by DXEngineering that not using the 80m resonator would work fine, hoverer that turned out not to be true in my situation.

Again my thanks to all who commented, I really appreciate the help and I learned a lot getting the antenna installed and operational. So, 73s to all and to all a prosperous New Year.
Don
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