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[WA5YOM]

David, KH6AQ......well....you certainly explain the efficiency loss.......when I compared my signal between my 40 meter bazooka...and the Greyline (listening to myself on RHR from the east coast)......I went from an S4 on the Greyline to S9 on the Bazooka......I only heard a faint barely readable signal on 80........I operated CWT this evening on 40 with the Greyline (it tunes the low end but not above 7.150)......it got out, but it was a challenge......I really haven't noticed much success...on any band.....for example...I tried FT8 on 24mhz....called cq for an hour.....no response....and there were tons of signals......had a local listen to my FT8 on 80....gave me a minus 21.....
What do you think about the balun I am using...I expect that if I break down and get the remote tuner......I still need it since the MFJ doesn't have a balanced input......In order to connect this into my system...... the TGXL will have to bypass this antenna.....it can do this through the 4o3A Antenna Genius.....

[W9IQ]

This is the Choke Balun I am using...

https://www.dxengineering.com/parts/DXE-MC20-1-1T

The balun has no technical specifications so it is difficult to judge its effectiveness. If you go the route of a remote tuner, I would keep a balun in place.

A remote tuner is an expensive investment. I would first measure the feedpoint impedance at the balun to ensure the tuner will be up to the task.

- Glenn W9IQ

[KH6AQ]

David, KH6AQ......well....you certainly explain the efficiency loss.......when I compared my signal between my 40 meter bazooka...and the Greyline (listening to myself on RHR from the east coast)......I went from an S4 on the Greyline to S9 on the Bazooka......I only heard a faint barely readable signal on 80........I operated CWT this evening on 40 with the Greyline (it tunes the low end but not above 7.150)......it got out, but it was a challenge......I really haven't noticed much success...on any band.....for example...I tried FT8 on 24mhz....called cq for an hour.....no response....and there were tons of signals......had a local listen to my FT8 on 80....gave me a minus 21.....
What do you think about the balun I am using...I expect that if I break down and get the remote tuner......I still need it since the MFJ doesn't have a balanced input......In order to connect this into my system...... the TGXL will have to bypass this antenna.....it can do this through the 4o3A Antenna Genius.....

The balun has no CM (Common-Mode) impedance specs and if you want to know what it is you will have to measure it. But what CM impedance is needed? We can explore this using EZNEC or any other NEC-based antenna simulation program.

While this antenna model is not an exact model of your antenna it does allow us to see roughly what CM choke CM impedance might be required. I believe the antenna is fed somewhere near the center with a coaxial cable routed down the bottom antenna tube. For my model a third antenna wire is connected at the feed point and is routed horizontally. This third wire represents the coaxial cable shield A load is placed in the third wire near the feed point and represents a CM choke. For a 67' wire at 3.5 MHz with the CM impedance (load) set to 5000 ohms the coax shield current is about 1/4 that of the antenna current. The antenna radiation pattern is distorted with the coax shield becoming a significant radiator. For a 134' wire the 5000 ohm choke has practically no effect. 5000 ohms at 3.5 MHz could be asking a lot from the balun you have. One way to reduce the shield radiation is to form a Pi filter at the antenna base like so:

COAX (inside antenna)--------CHOKE---ATU---GND---CHOKE--------------------coax to the shack

The GND can consist of a short GND rod only. Of course a longer GND rod with a number of short radial wires would be even more effective for reducing coax shield current.

[WA5YOM]

Well....the documentation on this antenna is a real chore to put together......in other words ,,,,the user has to "decode" the multitides of documents on the site......there is no one signal document that goes from start to finish......
So.....with that in mind.....
This is a Floating ground OCF......which they specifically say...not to ground the antenna...but obviously the remote ATU has to be grounded.....I am assuming (because I haven't looked at the atu documents), that the ground you are discussing is installed connected on the shack side of the tuner? Otherwise, if it is on the antenna side, then the antenna is not a floating ground......correct?
The feed point is at an insulator between the first section and all the remaining sections, which is about 4 ft up from the base....450 ohm twinlead goes up the first section, then exits at the insulator that isolates the first 4 ft section (approximate length) from the rest of the vertical...the plus side of the twinlead (center conductor of the coax) is connected to the upper portion of the vertical, the negative goes to the lower section (shield of the coax)......the twin lead connects to the correct terminal of the choke balun, then the outlet side of the choke balun connects to the pl259, then to my TGXL........I can provide exact dimensions....if that helps.....
So, to create the pi network, it appears that another choke/1;1 balun is connected at the shack.....specifically where is it connected in the shack?....I have a antenna switch.....insert the choke on the antenna side of the switch.....or can I insert all the way back at the radio output connection...thus the choke would be ahead of all my other antennas on that antenna port of the radio....
Thanks,
Tim

[WA5YOM]

I have a MFJ-226...I believe it can check the feedpoint impedence.....but I have not used it for that function before.....I just used the single freq. mode to check swr....

[KH6AQ]

Tim, thank you for the detailed description of the antenna innards.

Yes on earth grounding on the shack side of the ATU. The balun (common mode choke) can be installed on the 50 ohm shack side of the ATU. The tuner then floats at antenna potential. With this balun placement the balun is working in a 50 ohm differential-mode environment. With the balun at the ATU antenna side the balun must handle a very high SWR. It should make no difference to the core. Placing the balun on the shack side is simpler when using an ATU that carries the DC power on the coax. Otherwise, the coax and the control cable must be routed through the ferrite core(s).

Adjusting the EZNEC model to place the RF source at the 4' point the antenna impedance at 3.5 MHz is:

1.7 -j2900 ohms when modeled in free space
16 -j2760 ohms when modeled with the bottom 1' above real GND of 0.005S/13 relative permittivity.

What this means is that ground loss raises the real part of the input impedance (1.7 ohms to 16 ohms). If this reflects the real installation the near-field radiation efficiency is about 10%. Note that the model does not include the 4-5 ft of ladder line.

An alternative to the OCF vertical dipole configuration is a vertical of the same height fed against earth GND and radials; changing things so the antenna is a continuous 28' fed against a 25 ohm GND. The tuner sees a more manageable 30 -j530 ohms to match. EZNEC shows the OCF version providing 2 dB more signal. My guess is 25 ohms or less base-referred GND resistance should be achievable with a dozen 25' radials.

[WA5YOM]

OK...so, the bottom section is about 11" above ground......so we are close enough on that part of the calculation....
I am still slightly confused on the final configuration....

antenna - my existing 1:1 choke balun as installed (I expect it must be installed here because the ATU has an unbalanced output and the ocf is balanced) - remote ant tuner (per Greyline antenna manufacturer, the remote tuning unit must be installed 6" above ground potential) attach shack side of atu to a ground rod - coax to shack - additional 1:1 choke balun - Antenna Genius antenna switch - TGXL - PGXL - Flex 6700

If this is correct, then my next question is, can I put the additional choke balun between the 6700 and the PGXL or where is the best physical placement per this diagram?

The other Antenna option is to modify the purchased design and convert to a grounded vertical with radials. correct?

But the OCF still has a 2db gain over the modified option?

In either case...the remote atu is absolutely necessary?

Am I following you correctly?

Thanks,
Tim
wa5yom

[KH6AQ]

Your balun at the ATU output will work and I would place the additional balun right at the ATU input to reduce feed line external RF current per the simulation: 
Antenna--balun--ATU--GND--balun------------shack



In simulation the OCF version is more efficient than the 28' vertical by 2dB at 3.5 MHz. However, my model does not include tuner and balun loss that may or may not be higher with the OCF version. For me the 28' vertical is a known quantity while the OCF is an oddity. It is easy enough to try the 28' vertical if you aren't satisfied with the OFC version.

In either case a remote ATU is needed if you are to avoid high loss in the 45' of LMR-400 coax. I have an MFJ-993BRT remote ATU you are welcome to borrow to test this out. You just have to pick up the shipping from here and back. USPS Priority mail flat rate shipping is $22.45. If it works and you like it I might be up for letting it go as it's a leftover from the 34' all band vertical.

[WA5YOM]

Wow....an incredible offer....
Sure.......can't pass that up.....
I would return...since I need full legal power tuner
Thanks....
Tim
Wa5yom

[KH6AQ]

Tim, I will get it in the mail on Monday to your QRZ address.

[WA5YOM]

thanks.....send me info on how to pay the shipping.........
paypal?

[RFRY]

... An alternative to the OCF vertical dipole configuration is a vertical of the same height fed against earth GND and radials; changing things so the antenna is a continuous 28' fed against a 25 ohm GND.  The tuner sees a more manageable 30 -j530 ohms to match. EZNEC shows the OCF version providing 2 dB more signal.  ...

But observe that the OCF vertical shown in my Reply 3 neither needs nor uses a wired path to/through conductors buried in the (lossy) Earth in order to radiate about 90% of the Z-matched r-f power at its feedpoint terminals.  The radiation efficiency of that OCF dipole configuration is only 5% less than from an antenna system having zero loss!

That ~5% loss results from the ESRs of the radiator conductivity plus the the matching network at the feedpoint terminals, both of which are included in the values shown in Reply 3.

That is the reason why the OCF dipole has greater gain than a vertical monopole radiator using a series path through the lossy Earth, to enable r-f current to flow on the 'grounded' antenna system.

[KH6AQ]

Thanks RFRY for the OCF info and let me run this by you. My EZNEC model (NEC-2) shows additional feed point resistance -- as well as decreased gain -- pointing to substantial GND loss when placed close to GND. The recommended installation is with the base 1' above GND.

Antenna: 28' long OCF, 3.5 MHz, modeled with Real GND, 0.005S/13.

Base height   Average Gain test  Real part of input Z
1'                 -11 dB                  16 ohms
2'                  -9 dB                   11 ohms
4'                  -8 dB                     8 ohms
8'                  -7 dB                     6 ohms
16'                -6 dB                     4 ohms
Free-space      0 dB                     2 ohms

A near-field radiation efficiency calculation using the free-space input resistance and the near-GND input resistances show near-field radiation efficiency approximating the EZNEC Average Gain test. For example, at 1' the near-field radiation efficiency is 12.5% which is loss of 9 dB.

RE = (2 ohms/16 ohms) x 100% = 12.5%

[KH6AQ]

Our models differ in the band and the feed point location. The actual antenna feedpoint is 4' (14%) from the base.

[RFRY]

Thanks RFRY for the OCF info and let me run this by you. My EZNEC model (NEC-2) shows additional feed point resistance -- as well as decreased gain -- pointing to substantial GND loss when placed close to GND. The recommended installation is with the base 1' above GND.

Antenna: 28' long OCF, 3.5 MHz, modeled with Real GND, 0.005S/13.

We are comparing different antenna systems here.  Below is a re-post of the one I included in Reply 3 of this thread.

The numbers now showing on the graphic apply to the text entries in the numbers listed below.

• The base of my OCF dipole is elevated 8" AGL.
• The "Efficiency" is the percentage of the Z-matched r-f power output generated by the tx PA that is present across the antenna feedpoint terminals.
• The far-field-only "Radiat-eff." value is not a measure of the radiation efficiency of the antenna system itself, because NEC is not accurately considering the complete radiated wave when the (NEC-defined) "near-field" analysis is excluded from the calculation.