by: Michael G. "Mike" McGuire, K3SNO
ANTENNA DESCRIPTIONFrom the MFJ AV-680 manual;
"Computer optimization of the AV-680 design yields the most efficient 3/8 wavelength electrical design with maximum gain and low angle of radiation for long distance communication. No traps are used to achieve nine band performance. The AV-680 is resonant on 6, 10, 12, 15 and 17 meters with individual 3/8 wavelength radiators. The center radiator resonates on 20, 30, 40 and 80 meters using parallel end-loaded Teflon wire coils. Capacity hats on these bands give wide 2:1 VSWR bandwidth and the antenna is kept to a height of 27 feet by the low inductance coils. There are no "tricks" or "mystery resonances" used for impedance matching on any band." |
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DETERMINE YOUR GOALS
Prior to embarking on this antenna project, I reflected on my objectives within the realm of Amateur Radio. After a break of over 30 years from the hobby, I was astonished by the advancements made during that period. Historically, I had been an avid Phone DX'er and a casual participant in contests. My interests appeared to remain unchanged, leading me to determine early on that the antenna system I selected would be primarily oriented towards SSB Phone operations and would aim to cover as many bands as possible. Given my limited space, the lack of suitable trees for hanging wire antennas, and a budget that did not permit the installation of a tower, I concluded that a vertical antenna system would be the most appropriate choice. A piece of advice from a seasoned ham (Elmer) from many years ago echoed in my mind: "Regardless of your budget, ensure that you allocate at least two-thirds of it to your antenna system." This principle, which emphasizes that the antenna system begins with the cable connected to the back of your radio, remains highly relevant today.
After reveiwing the many antenna systems out there, and immediately by-passing any which would require a radial ground feild, I determined that the best option for me would be the Hy-Gain Patriot Plus AV-680 Antenna. This antenna covers 6m and 10m through 80m, has a very low radiation take-off angle, and can hadle full legal limit power of 1500w (US). It also does not require ground field radials, which is very advantageous for my situation.
Because of the nature of the AV-680 tuning, the mast would have to be a tilt-base style with a 2 foot base stub above ground, a Tilt-Base unit installed on the base stub, and a 16 foot (absolute minimum is 8' according to MFJ) Schedule 40 aluminum mast pipe on top. The Tilt-Base, which I am convinced is essential for this type of antenna, facilitates easy raising and lowering of the antenna for tuning purposes. The final installation would create a mast height of 18' from gound to top, on which to install the AV-680. At 27' tall, the AV-680 would be installed at a height of 45' from ground to antenna tip.
Next I decided the use of a remote tuner in conjunction with low-loss, high quality LMR-400 coax. Together they are very advantageous for getting the most signal into the ether. Rather than a shack tuner, a remote tuner optimizes the already 50 ohm coax and augments the AV-680's Common Mode Current (CMC) choke (inside, and part of, the antennas matching unit) by having a secondary CMC choke to further reducing the possibility of CMC and EMI back to the shack. In conjunction with a well tuned antenna, a remote tuner would only be required to maintain a very small correction for the receiver, if any at all on most bands, and accomodate the AV-680 bands which have a small 2:1 bandwidth. Reveiwing many remote tuners, the choice quickly became the Legal Limit IntelliTunerTM, Remote Automatic Antenna Tuner, MFJ-998RT (MFJ-998RT Manual - MFJ-998RT Schematic) Operating between 1.8-30 MHz, the ((998RT can handle up to full legal limit 1500w (US). Both input and output connectors on the MFJ-998RT were converted to N-Female flange connectors to accomodate hte N-Style LMR-400. Incidentally, the MFJ-998RT will also tune 6m if the antenna is tuned below 3:1 VSWR in the area of interest.
Reveiwing the ARRL's best practices documentation, installation of a local static/electrical safety ground field consisting of a 4-spoke array of 8' ground rods would be installed 8' feet away from and around a centrally located 8' copper ground rod adjacent to the mast base. The four peripheral rods are bonded to the central rod using #2 AWG Black Stranded Copper Wire. The central rod is bonded to the lightning arrestor ground rod as well as the residence electrical service system ground rod, using the same type #2 AWG wire.
The Lightning Arrestor enclosure would be installed on the exterior of the residence adjacent to the shack room. Its ground rod, located immediately below the enclosure and bonded to house electrical ground rod, provides the ground source for the shack ground via 2"wide copper strap. The coax pass-through into the house would also contain the shack ground.
STATION/ANTENNA SYSTEM OBJECTIVES
a. No antenna ground required
b. Cover as many bands as possible
c. Build for legal limit power
d. Use of a remote tuner
e. Use high quality LMR-400 coax
f. Install lightning protection at coax entry point
g. Install an integrated all Static/Safety ground systems to building electrical ground
h. Tune the antenna to the center of the Phone DX slots on all bands
Secondary considerations included;
1) No ground plane required (space limitations),
2) Ability to use full legat limit power (i.e.: 1500w US) if required, and
3) All system components maximizing low power level capability with minimal ratiation take-off angle.
Therefore I chose both the MFJ-998RT Remote Inteli-Tuner and the Hy-Gain AV-680 antenna because each is capable of handling 1500w PEP and fit my other criteria. The AV-680 has a very low take-off radiation pattern ( <10 degrees) and only required a minumum of 8' off the ground. Also, several coaxial cables were considered, chief among them RG213/U Coax and LMR-400 Coax, and the final choice was determined to be the LMR-400 because of it's considerably better capacitance value, better sheilding, and much higher velocity factor. True, LMR-400 cannot handle the RF power that RG213 can (3.3 kW @ 50 MHz vs. 16 kW), but the higher velocity factor (84% vs. 66%), the better sheilding, lower capacitance value (23.1pF/ft vs. 32.2 pF/ft ), and considerably lower attenuation loss below 50MHz (0.7 dB/100 ft vs. 1.2 dB/100 ft, is more than ample reasons to warrant the additional cost.
My past experience with RG213 had been that it is more susceptable to picking up RFI from its environment, and therefore a higher S-level noise is normally seen on the rig with it's use. Not good for weak signal work. I have found that LMR-400 coax exhibits a much lower noise level than RG213, which as anectdotal as this might be, it makes me believe it's due to the double sheilding and 100% tinned double braid coverage built into the LMR-400 cable. I have also noted that though the LMR-400 specification says 0.7 dB attenuation @ 100 ft, in actual use I am seeing roughly 0.35 dB loss at 14.225 MHz in a total coax run of 95 ft through 6 connections. That translates to 98+ watts getting to the antenna tuner feed point for a 100 watt output from the transmitter. All antenna tuning was performed at the end of the 15' LMR-440 patch cable coming down from the antenna to the MFJ-998RT. Tuning at the antenna ensures the frequency of choice will radiate the most effectively due to the antennas natural resonance. The tuner only makes the radio happy, it does not tune the antenna. Keep that in mind.
ANTENNA MAST INSTALLATION
The initial task was to install the base pipe. I opted to excavate a hole measuring 1'x1'x2' deep. Subsequently, I inserted a 7' heavy-wall steel water pipe, aligned in two axes, into the bottom of the hole, leaving 2' above ground level. I then filled the hole with three bags of SAKRETE to complete the base pipe installation, allowing it to set for two days. After the setting period, I mounted an MFJ-1903 Universal Tilt-Base Mount on top of the mast base and attached a 16' section of Schedule 40 heavy-wall aluminum pipe to the upper half, resulting in a total mast height of 18 feet above ground. Once the 27' antenna was assembled and installed, the tip of the antenna would reach 45' from the ground. Additionally, five 8' ground rods were driven into the earth, with one placed near the base pipe foundation and the other four positioned 8' to the left, right, in front of, and behind the base pipe. The four perimeter ground rods were connected to the base pipe ground rod using #2 AWG stranded copper wire, while the central rod was bonded to the aluminum upper mast pipe.
ANTENNA ASSEMBLY
If you have never seen or assembled one of this type of antenna, the instructions provided by MFJ leave much to be desired (Link to the latest AV-680 manual). After reading, re-reading, and reading again, I thought I was ready to build. I initially assumed this antenna would be up in the air in a few hours. Not the case at all. in hind-sight, with better instructions what should have taken about two or three hours took an entire day. The process of installation and tuning eventually consumed two full days just to get to the point of having the upper bands at least usable, but nowhere near final tuned. If I had to build one again, I'm sure it would be a three hour job to get it to the point of testing/tuning. Experience is everything.
SIDE NOTE BASED ON ONE YEARS EXPERIENCE WITH THIS ANTENNA: All mating metal parts should be lightly coated with an electrically contuctive grease to prevent oxidation and to ensure long-term conductivity between parts. I recommend NO-OX-ID 'A SPECIAL' by SanChem, Inc. (NO-OX-ID on Amazon). This includes all interconnecting tubes, counterpoise radial parts, wire connections, and capacity hat radials. EVERY metal-to-metal mating surface ... lightly coated. Ask me how I know. LOL
I recommend assembling the antenna in a horizontal position, such as laying it across saw horses, starting with the 1-1/2"x12" base tube (BA) and progressing to the top Radiator Section-5/8"x36" (BF). I successfully completed the assembly of the entire 17'-6" base section up to this stage. After constructing the main mast, I proceeded to install the four 3/8λ radiator Base Brackets for the 6m, 10m, 12m, and 17m radiators. It is crucial to position these brackets as accurately as possible, measuring from the bottom of tube BB, as indicated in FIGURE C of the manual.
However, the manual does not clearly state that the measurement reference point shifts from BB during the assembly process, which could result in significant dimensional inaccuracies. FIGURE E illustrates the dimensions from the bottom of the BB pipe to the radiator base mounts. When installing the radiators, the instructions suggest measuring from the base mounts to the tip of the radiator. A more effective approach would be to use a 25' tape measure, sum the two measurements while maintaining the same reference point (the bottom of BB), and adjust the tuning tip to this total length.
I advise that you combine the dimensions for each band from FIGURE C with those in FIGURE E to determine the length for the radiator tip. This method ensures greater accuracy by using a consistent reference point for all measurements, thereby minimizing the risk of assembly errors or inaccuracies in radiator positioning. Additionally, ensure that the Stub Insulators are positioned both rotationally and dimensionally with precision. Any misalignment could result in the radiators being misaligned with the main radiator pipe, potentially leading to issues later due to the bending moments of the antenna.
Upon reaching the 20M-80M capacity hat section of the antenna, I completed its assembly and set it aside until the main radiator assembly was secured to the mast. At this stage, I attached the assembled base of the antenna to the tilt mast, supporting it with a saw horse so that the antenna's end was elevated approximately 4 to 5 feet off the ground. Next, I connected the 20m-80m capacity hat to the top of the main radiator. I then attached a 15-foot LMR-400 coax cable to the base of the antenna, specifically to the matching box, and applied waterproof rubber tape to the connection, followed by an additional layer of electrical tape for enhanced protection. Before elevating the fully assembled antenna for the first time, I installed the MFJ-998RT Remote Intele-Tuner on the upper mast, about 5 feet above the ground. Subsequently, I ran a 75-foot section of LMR-400 from the Remote Tuner to the lightning arrestors located on the side of my house. It is important not to connect the antenna feed line or the main feed line to the tuner at this point, as the antenna will be tuned from the bottom of the 15-foot LMR-400 later on.
ANTENNA TUNING OBJECTIVES
One of my primary objectives was to install an optimized antenna system which exhibited the least amount of signal loss combined with a tuning scheme specifically targetting the Phone DX portions of each band of operation. I used the table below as the tuning target frequencies. Depending on what you will be focusing on in your radio endeavors, your target frequencies may be differnet. I am going to be very honest here. I am rather more precise than is necessary when it comes to tuning. I am being as precise as I can to get the most out of my system. To me it matters. In the "Reasoning" section I added the frequencies that a General Class operator might want to use, just for reference.
Keep this in mind as you read the following paragraph; "Every antenna is only "optimally resonant" (or as resonant as it can be by its design) on exactly one frequecy within its band of operation. Every other frequency within that band is a 'trade-off'" of sorts."
It is important to understand that each antenna has a specific resonant frequency; however, it can generally perform effectively across a range of frequencies, provided that its 2:1 VSWR bandwidth is sufficiently broad around the resonant frequency. Ideally, we want the antenna to operate over as wide a bandwidth as possible to ensure optimal performance near its true resonant frequency. Typically, the antenna achieves maximum efficiency—radiating the highest power with minimal loss—when the VSWR is at its lowest achievable value, assuming the antenna presents an approximate 50-ohm impedance at resonance. Additionally, it is crucial to note that VSWR represents the relationship between the radio/feedline impedance (50 ohms) and the impedance observed at the antenna connection, reflecting the degree of signal reflection within the transmission line. If the antenna shows a 150-ohm impedance at its feedpoint at resonance, this indicates an impedance mismatch, resulting in a 3:1 VSWR. Such a condition can be detrimental to your radio unless a tuner or a matching transformer is employed at the feedpoint.
So assuming the design of the AV-680 is "good enough" for its purpose, my first decison was how, or more appropriately "where", in the band I wanted to "tune" the antenna, and what am I trying to accomplish by putting it there. Being an Amateur Extra Class operator, I have the full spectrum available to me. But, most DX happens at or just above the lowest General Class licensee frequency. Being that I am primarily interested in Phone DX and Phone Contesting, my objective became; "Tune the antenna to the middle of the Phone DX portion of each band. That is normally in the area between the top of the band and the lower end of the General Operator phone frequencies. This makes the antenna most resonant around those primary DX locations, and therefore radiating the highest effeciency of power, with no or little re-tuning required when switching bands during DX'ing or a contest. This equates to a fast band-switching system with far less work involved before you can start transmitting. I am fully aware that I could have tuned each band to the bottom of the band and allowed the MFJ-998RT Remote Inteli-Tuner to tune the entire band capacitively, but that defeats my objective of maximizing signal efficiency and band switching speed.
TUNING PARAMETERS USED
| BAND | Band Start | Ph. Bottom | Tune Target | Band End | Reasoning |
| 6m | 50.000 | 50.100 | 50.125 | 54.000 | 50.125 is the US Calling Freq. |
| 10m | 28.000 | 28.300 | 28.550 | 29.700 | Middle of the DX Slot |
| 12m | 24.890 | 24.930 | 24.960 | 24.990 | Phone Band Middle |
| 15m | 21.000 | 21.200 | 21.325 | 21.450 | Phone Band Middle (General Lic.: 21.360) |
| 17m | 18.068 | 18.110 | 18.140 | 18.168 | Phone Band Middle |
| 20m | 14.000 | 14.150 | 14.250 | 14.350 | Phone Band Middle (General Lic.: 14.287) |
| 30m | 10.100 | n/a | 10.125 | 10.150 | Band Middle |
| 40m | 7.000 | 7.125 | 7.210 | 7.300 | Phone Band Middle (General Lic.: 7.240) |
| 80m | 3.500 | 3.600 | 3.800 | 4.000 | Phone Band Middle (General Lic.: 3.900) |
NanoVNA-H4 TEST EQUIPMENT SET-UP
1. With the stimulus range set to 1 MHz to 60 MHz, the NanoVNA-H4 was calibrated with known good standards (Open, Short, and a perfect 50 Ohm Load). Once calibrated, the calibration was verified using the same standards. All calibrations and verifications were performed with the same extension cables install to minimize cable influence of subsequent measurements.
2. All scans were performed using a Linux-based laptop and the NanoVNA-Saver application, and the latest firmware on the NanoVNA-H4 device. The NanoVNA-H4 was connected to the laptop via USB cable.
3. An initial verification run was performed on each band and screen shots of the scans were saved.
4. Tuning consisted of determining the frequency at which the lowest VSWR was achieved on each band, and moving the tuning stub accordingly per MFJ's chart in the manual. These numbers proved to be reasonably accurate, and a spreadsheet was created to assist in calculating the tuning changes based on frequency movement shown in the manual. The spreadsheet output was adjusted to give me dimensional moves in 1/16" increments (fractionally). Made it a lot easier to use a ruler. LOL
5. 6M to 17M were tuned first, starting with 6M, and working up by bands to 17M. Once all 5 bands were tuned (as indicated in the respective NanoVNA-Saver images below), then the capacitive radials were adjusted in length to tune the 20, 30, 40 and 80M bands one band at a time until each band was within the target area. Be sure to have the 80m coil tapped at the very top initially, and tune the radials for the bottom of the band. Once 80m is tuned to 3.5 MHz, then move the tap to acheive the 3.800 MHz final tune. This allows for any future re-adjustment without having to mess with the capacitive radials again.
4. Once all bands were roughly dialed in, each was cross-checked using a Daiwa CN-9011H3 meter, an MFJ-894, the internal VSWR meter in the Yaesu FT-991, and an ICOM IC-7300. All agreed to within 0.2 VSWR of the NanoVNA-H4 readings.
5. Again, my objective in tuning each band was to place the minimum VSWR reading at the middle of the common Phone DX slot. As stated earlier, this enables me quickly change bands during DX'ing or Contesting, and I do not need to retune more times than not when changing bands or frequencies. I later found that 6m to 20m required no tuning at all, unless I was really trying to eek out every ounce of energy.
NanoVNA-H4 SCAN IMAGE DESCRIPTION
Here are the "final" NanoVNA-H4 scan images for each band, ranging from 6M to 80M, along with the 160M band included out of curiosity. Although the antenna is not specifically designed for this band, it does not imply that it cannot function on it. I have successfully made several NVIS contacts in the lower segment of "Top Band" (160m) and can adjust the entire bandwidth using the MFJ-991RT Inteli-Tune Remote Tuner.
All of the scans follow the same formula;
1. The MFJ-998RT Inteli-Tuner was powered on and by doing so it is in By-Pass mode. We are testing VSWR through the tuner, and therefore through the by-pass relay within it, with no inductance or capacitance engaged.
2. Each US Amateur Radio band is scanned, one band at a time (6, 10, 12, 15, 17, 20, 30, 40 and 80M), and a screen shot taken of the results.
3. The colored Markers; RED, GREEN, and BLUE, indicate as follows; RED = Beggining of the Phone Band, GREEN = Target Tuning Frequency of lowest VSWR, and the BLUE = End of the Phone Band.
4. The RED horizontal line displayed in the top two graphs indicates the 2:1 VSWR Threashold. Anything below 2:1 VSWR is considered usable, and also indicates the usable bandwidth where the VSWR scan crosses that line. The lower left graph also shows the 2:1 bandwidth, but in Smith-Chart format, as a RED circle.
5. S11 indicates the frequency at which the lowest VSWR occurred, and what that VSWR value is (ignore S21, its not used for antenna testing).
6. Marker 1, Marker 2, and Marker 3 areas indicate all of the spectral data for the antenna at that frequency marker point. Most of it is not used for VSWR tuning, but i like knowing what those values are for other reasons. The VSWR value is good to know though. Also of note is something about Marker 2. Due to the sensitive (read that small and fractional) nature of stub tuning, there's a point at which you arrive at a tuning requirement of 1/16" or less. Attempting to tune to that level of precision is not only un-necessary, but it also will not yeild any measurable difference in your reception, your output power, or the DX stations reception of you.
7. When reveiwing the images below, the most important point is the S11 VSWR Graph and the Return Loss graph. Those are the actually tuning response of the antenna across the entire band.
PERFORMING THE TUNING
The AV-680 manual includes a chart displayed below, modified slightly to present all values on a per-inch basis. I discovered these values to be highly precise, prompting me to develop an Excel spreadsheet to determine the necessary tuning adjustments. By conducting a scan with the NanoVNA-H4 and entering the frequency at which the lowest VSWR was recorded, the spreadsheet efficiently calculated the required adjustments in fractions of an inch to achieve the desired frequency.
| BAND | TRIMMING EFFECT |
| 80 m | 10 KHz per Inch |
| 40 M | 17 KHz per Inch |
| 30 M | 50 KHz per Inch |
| 20 M | 100 KHz per Inch |
| 17 M | 120 KHz per Inch |
| 15 M | 100 KHz per Inch |
| 12 M | 300 KHz per Inch |
| 10 M | 200 KHz per Inch |
| 6 M | 600 KHz per Inch |
Click the images below for a larger veiw.
6 MeterThe following images are NanoVNA-Saver software VSWR scans of the final tuning of my Hy-Gain Patriot-Plus AV-680 9-Band 3/8λ Vertical Antenna using a NanoVNA-H4." |
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10 Meter |
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12 Meter |
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15 Meter |
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17 Meter |
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20 Meter |
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30 Meter |
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40 Meter |
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80 Meter |
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160 Meter(antenna is not designed for this band, but can be used NVIS when tuning with the MFJ-998RT Inteli-Tune Remote Tuner) |
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THE SYSTEM IN OPERATIONPreviously I discussed the how I did things. Now I'd like to describe the what and why. Understand that I was making every attempt to avoid having to install a ground field below the antenna, hence why I chose the AV-680 for it not needing one. Second I wanted a "system" capable of running full-legal-limit of 1500w. The Av-680 is a 1500w antenna. Another choice along the same lines is that I chose to install a remote tuner because of the increased efficiency gained (vs. an ATU in the shack) by having the tuner at the antenna. The MFJ-998RT handles 1500w. If you're going to create a 1500w system, you must use 1500w coax. That's where the LMR-400 comes in. I could have installed LMR-600, but that would truly be over-kill and not enough of an advantage over the LMR-400 to justify the added expense. The entire system displays less that 0.7 dB of loss at it's worst tuning location. To put that into perspective, we're talking a loss of ~15w of a 100w transmission. Not bad at all for a worst case scenario. At the best locations it's running about 0.4 dB or less for a loss of 10w @ 100W output. So how does it all work in actual practice? A very valid question. I installed the antenna in the end of January 2024. I run either an FT-991 or an Icom IC-7300 radio at only 100w, no amplifier. I have an amplifier but I have only used it once for testing and never turned it on again. As of this writing in September 2024 I have made over 3500 QSO's, been awarded a Phone-Only DX Century Club (DXCC), a Phone-Only Worked All States (WAS), competed in several contests finishing relatively high in the rankings. The antenna workls quite well in my opinion. I've seen so many reveiws that degrade the performance of this antenna and now in hind sight I suspect those poor reveiws are due more to incorrect assembly, poor tuning, or the the owners lack of general antenna theory knowledge. The antenna certainly performs well beyond my expectations and has been phenominal for DX. TO BE CONTINUED ... REFERENCES MFJ-998RT Manual MFJ-998RT Schematics Hy-Gain AV-680 Instruction Manual LMR-400 Spec. Sheet NO-OX-ID "A-Special" Grease |
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