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  Experimenting With 6 Meter
Ground Plane Antennas

By John Haserick W1GPO
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Background:

We have been experimenting with a number of 6 meter repeater antennas over the past 40 years, and would like to share what has been learned. Herb and Glenn Kreckman of Kreco Antennas have been very generous with parts, antennas, and advice on their coaxial dipoles. We have built several repeaters using the DB-212 side-mount dipoles and several with a special version of the ASP-600 ground-plane. The article includes photos of a special version Kreco Co-Plane, with excellent lightning protection, bandwidth, and mast decoupling with about the same gain as a 5/8 ground-plane, and a 3 dBd version of an ASP-600B (30-40 MHz) converted to a 6 meter 5/8 over 3/8 ground-plane.

Click on any photo for a larger view.

Duplexing off one antenna on 6 meters is more challenging than on the higher frequencies for several reasons:

  1. The inductive near field is much larger, which can create coupling to household wiring. For example, with a 5/8-wave ground-plane with eight decoupling radials below the coil and ferrites on the coax feed-line at the antenna (a Dominator 5/8-wave from Norwalk Electronics), chimney mounted at 30 feet above ground, radials three feet above the chimney, there was no problem running 100 Watts simplex or on receive only, but when duplexing, the garage compact fluorescent bulbs created noise from rectified RF, the garage doors themselves caused "scratchies", etc., making the repeater useless. In another location, using the equivalent of the Hy-Gain V6R 3 dB ground-plane with radials at 62 feet at the top of a tower, and the house 75-100 feet from the tower, we had the same problem with certain fluorescent ballasts, garage door springs, electric razor charger, and even the neighbor's light dimmer on certain settings, but the repeater was more usable, even though it experienced a few dB of de-sense when transmitting.
  2. The antenna may have downward secondary lobes either from not enough height above ground, or the antenna design itself. The Dominator 5/8-wave, even though the charts show the radiation pattern main lobe at maybe 10 degrees above the horizon, and no downward lobes, did in fact have a 45-degree downward RF field peak circumferentially around the outside of the house, measured with an RF sensor. This RF peak disappeared when the antenna was raised so the radials were at 45 feet above ground. I conclude that a 5/8 ground-plane on 6 meters needs to be about 45 feet above ground and other conductive objects to minimize ground losses and keep the RF from mixing. The 5/8 over 3/8 V6R ground-plane has both a 60-degree down secondary lobe, and slight downward tilt of the main lobe, therefore it would experience problems with the neighbor's wiring as well.
  3. Coronal discharge off top-mounted antennas, if not next to a higher antenna. This is a real receiver killer whether duplexing or not, and occurs even when the entire antenna is wrapped with Scotch #35 tape for precipitation static. It can also reduce the chance of lightning hitting the antenna, which happened with us: a tree 50 feet from the antenna and lower in height took a direct hit, splitting the bark its entire length, but the repeater, even with GaAsFET pre-amp kept on working unharmed. Before we installed the Static Buster there were several occasions when we heard "the squeal," indicating a large charge buildup, and the repeater was often desensitized by coronal discharge buildups.
  4. Quality feed-line and connectors are important. Use N connectors with captivated pins so they do not pull out, or DIN connectors outside. UHF will work if you can keep the water out. We like thick heat shrink tubing with internal glue to bond the connector to the feed line, Scotch #88 tape, then non hardening butyl rubber and then more tape for outside connections. Indoors, BNC connectors do not make a good shield connection, especially if reconnected often. If you have them, spread the fingers on the male shield so they fit tight when connected, and try to prevent movement that can re-loosen the shielding. Stay away from LMR type coax with the aluminum shield when duplexing as it is noisy. The best choice is Heliax wherever you can. There are usually lots of good Heliax deals on popular auction sites.
  5. Gain. Almost all antennas are overrated for gain. If you come across a commercial two-way antenna where its rated gain also displays the symbol "RS-329," that means it actually has the stated gain AT THE HORIZON and also meets certain durability standards. A 6 meter vertical antenna must have at least 18 feet of active antenna to have 3 dB gain over a 1/2-wave dipole, and even then it might not be at the horizon, or may have coil losses, etc. A good example is the 1/4-wave monopole ground-plane, which is very popular on low-band because of lightning protection. It is rated as "unity" gain but never has the RS-329 symbol because it has an upward beam-tilt and therefore something like -1.5 dBd at the horizon. Several models have the horizontal radials longer than 1/4 wavelength to help lower the radiation angle a little, but a good 5/8-wave ground-plane like the Dominator (a nicely modified Sirio 827 CB ground-plane) has noticeably better performance, but the #10 copper wire in the base coil could be damaged by lightning.
  6. Capture area. This is very important on 6 meters for several reasons. It noticeably reduces phasing nulls and dead spots to have more capture area, and more signal gets to the receiver to offset feed-line and duplexer losses. A 1/4-wave ground-plane with horizontal radials has the least capture area of any commercial base antenna.
  7. Grounding, bonding and guy wires. Antennas like the DB-212 side-mount dipoles, with their element about nine inches from the tower, minimize the backside RF null by coupling energy into the tower. That means everything connected to the tower including feed-lines from other antennas all the way back to inside the radio room, and the guy anchor rings next to the ground are sources of RF mixing noise. Any unterminated coax in the radio room with an antenna still on the tower will emit RF into the room, so it needs to have a shielded cap over the connector. The guy anchor-guy wire connection, as well as the turnbuckles need to be jumpered across with stranded copper, No-Alox, acorns, ground clamps, etc. Nothing bare against bare should rub, and for 6 meters, try to get the bonding within six inches of the moveable or RF weak joint that you are bonding across. Try to position antennas on the tower above where the guy wires attach, and probably at least nine feet from an above guy wire at any angle. We installed a set of DB-212 dipoles on the legs of a guyed tower with the lower dipole 4-5 feet below and 4-5 feet away from a guy wire. The dipoles had 1.0:1 SWR on a similar Rohn tower that had no guy wires. The SWR was something like 1.7:1 on the guyed tower. Even face mounting the dipoles further than nine inches out from the face only halfway improved SWR. Must have been also pattern distortion and RF directed into the ground from that guy wire.

Since most repeaters are not disconnected when an electrical storm is near, the tower ground, power line neutral ground, telephone, and cable grounds all need to be connected outside the building with probably at least #4 copper. I have also heard stories of the neutral being lost at the pole with the 120 volt lines still connected, so add a few good 10 ft ground rods at the service entrance for good measure, as no breaker box surge protector will protect against the loss of neutral!

Some 6M Antennas To Avoid For Duplexing:

  1. Ringo. Because it is voltage fed, it is fed at a high impedance point, which sets it up to be easily detuned by water to say nothing of ice. If the mast at the ring is at a 1/4 wave multiple of RF ground, half the RF from the antenna will flow into the mast and feed-line. That is why the Ringo Ranger has decoupling radials and bonds the coax at a voltage null.
  2. Any antenna where the loading or phasing coil is below the decoupling radials. This will also feed RF into the mast/feed-line. One such example is the Diamond DP-GH62.
  3. Diamond V2000, or Comet GP-15. This only has one radial for 6M, so there's not enough mast decoupling.

Some Recommended Antennas For Duplexing:

  1. Dominator 5/8 Ground-plane (from Norwalk Electronics, essentially a converted Sirio 827). Excellent decoupling, especially if you add a few ferrites on the feed-line just below the connector. (To show how good the decoupling was, there was no problem using a UHF four bay antenna with a Winegard LNA200 preamp mounted ten feet down the mast and 100 Watts on 6 meters at the antenna.) The vertical element is very strong; the radials the weakest part. Get the plastic radial ring to help strengthen the radials. It also has good bandwidth.
  2. DB-212 side-mount dipoles (used conversion). Wrap in vinyl tape for precipitation static protection. Great SWR bandwidth. Not good for omni on a wide tower; place on just one leg on wide tower. For two dipoles, the UHF tee connector needs tape plus butyl rubber plus #88 tape, because water gets in otherwise. It's very durable with end supports, no matter how much rime ice. DB-8 feed-line with polyethylene jacket, solid center conductor, and flooded braid lasts forever. The length of DB-8 from dipole to connector needs to be exactly the same length on both dipoles, but can be longer than 3/4 wavelength, and dipoles longer than one wavelength center-to-center without loss of gain, as long as the bottom of the lower dipole is at least 60 feet above RF ground.
  3. Antenna Specialists ASP-600 Ground-plane (no longer made, so used only). Either a 5/8 or 5/8 over 3/8 configuration. The 5/8-wave doesn't have as much gain or decoupling as the Dominator. The 5/8 over 3/8 version ties with the Hy-Gain V6R with the most gain at 3 dBd. It's more rugged than the Dominator or Hy-Gain V6R.
  4. Hy-Gain V6R (MFJ). This had the best mast decoupling at 20 dB. It has the same theoretical gain at 3 dBd as the ASP-600 5/8-3/8, but is not as rugged; the mast needs beefing up, and enhanced weatherproofing is a good idea.
  5. Co-Plane CP-41A (Kreco). This has the best lightning protection, is very well made, rugged, and has excellent mast decoupling and gain. The capture area is about the same as a 5/8-wave ground-plane. It can be modified for wider bandwidth. They are a great company to work with directly for custom applications.
  6. Monopole 1/4-wave ground-plane (a used Celwave or DB products is the best). It has negative gain at the horizon, but is rugged, lightning resistant, and has good bandwidth.

Converting The ASP-600 To 5/8 Over 3/8 GP For 6M:

The Antenna Specialists ASP-600 half wave to 5/8-wave ground-plane came in three frequency splits. The two most common were ASP-600B (30-40 MHz) and ASP-600C (42-54 MHz). On 6 meters it is more of a 1/2-wave ground-plane, so it has somewhat less gain than at 40 MHz.

It just happens to turn out that the ASP-600B coil is 1/4 wavelength on 6 meters, so it makes a great 5/8 over 3/8 center fed antenna with no modifications to the coil. The ASP-600C coil can be replaced by making a new one to be 1/4 wavelength. This requires a way to mount the antenna for tuning, a good antenna bridge showing reactance, and feed line close to odd multiples of 1/2 wavelength electrically for best tune-up.

We have converted three ASP-600Bs and one ASP-600C antenna. All outperformed any 5/8, 1/2, or 1/4-wave antenna, and have stood up to at least 1/2 inch of ice. The first one had two sets of four sloping radials, similar to the V6R. It was mounted at the top of a 100-foot tower. The site was high so it did have some coronal discharge desense, but no duplexing desense. We moved the lower four radials up to be between the upper set on the same hub, so now there are eight radials on the top hub. We had no change in field strength 25 miles away, and still no duplexing desense, so future versions just used one set of four or eight radials. The eight radials gave a little more bandwidth, with zero ohms reactance and 1.0:1 SWR at the resonant frequency.

6m-ant-pix/1.jpg

This is not a good antenna to duplex from a house, unless it is at the 100-foot level, because the 60-degree downward secondary lobe, which is probably down about eight dB from the main lobe, would be aimed into household wiring. A better antenna would be the Dominator 5/8-wave, or Kreco Co-Plane at least 45 feet above the attic floor.

The ASP-600 is mounted on 1-1/4 inch pipe with NPT into the bottom of the base. The three horizontal ground plane radials are no longer attached to the base, but could be reused instead of the stainless steel whips attached to the whip hub from Kreco, which has #3/8-24 threads. Schedule 80 pipe is recommended, which can get quite heavy for possibly more than 10 feet, plus the threads can rust even with No-Alox, so we went with 6061 T6 aluminum from DX Engineering and Online Metals, had a machine shop reduce eight inches of the 6061 aluminum 1-1/4 pipe to fit inside the tubing, and Glenn at Kreco enlarge the hub to fit just over the tubing.

6m-ant-pix/5.jpg

We used a drill and tap to make the threads to join the sections with screws and bolts.

6m-ant-pix/3.jpg

6m-ant-pix/8.jpg

6m-ant-pix/4.jpg

Here's a photo of the complete antenna without its whip radials. The upper element's upper hose clamps were replaced with screws, with Scotch 35 tape over the entire length. Grey tape might have been a better choice to blend better with the sky.

6m-ant-pix/7.jpg

Aluminum 6061 T6 schedule 80 pipe is not quite strong enough at about 10 feet to support the antenna in high wind. Permatex Ultra Blue RTV silicone gasket maker (auto parts) is a superior sealer, where needed.

6m-ant-pix/6.jpg

It is recommended to unscrew the top aluminum sealing ring on the fiberglass base coil assembly, remove the three bolts at the lower base portion just below where the radials went, and pull up the fiberglass housing to check the coil area for cleanliness. Clean and tighten the connection where the screw holds the coil to the aluminum tubing, add a tiny amount of No-Alox to just the aluminum-copper interface, then epoxy the cleaned, re-tightened joint.

Approximate substitute coil specs to convert the ASP-600C are about 50 inches #10 bare copper, 8-1/2 T, 1-3/4 inches OD, tapped at 3-3/4 T from bottom, coil length 2-1/2 inches.

Kreco Co-Plane Modification:

6m-ant-pix/b.jpg

In comparison to the 1/4-wave monopole ground-plane, 1/2-wave coaxial dipoles have less 1.5:1 SWR bandwidth. This makes duplexing, with TX and RX frequencies one MHz apart, less efficient because of higher SWR. The Kreco shunt-fed 1/2-wave coaxial antennas are fed by an inductive loop between the inside of the skirt and the mast inside the skirt. This means a return element can be added as on the monopole ground-plane from the top down to the middle where the skirt starts. By making the space between the original element and new return element about the same as on the monopole ground-plane, SWR bandwidths become equal, because the coaxial skirts are already wide, and not the limiting factor.

This photo shows the top element with the ferrule welded to a CP-41A antenna.

6m-ant-pix/c.jpg

These two photos show the return element ferrule fastened to the narrower diameter skirt hub of a CO-35A antenna using three #10-24 screws, each 1 1/4 inches long.

6m-ant-pix/9.jpg

6m-ant-pix/a.jpg

A new upper element and ferrule are required from Kreco. I do not recommend taking the easy way out and using the plated steel ferrule clip Kreco uses in their monopole ground-plane for duplexing, because a few years down the road, corrosion may set in, and "scratchies" may appear on the repeater. Instead, either weld or fasten with, say, #10-24 screws as shown. Welding is better, but this means taking the antenna apart. The problem with the screws is that the return loop length is not adjustable, as the holes go through the solid 3/8-inch return element rod. Note the two parallel flat edges, one against the antenna, and one for the screw heads. This was a CO-35A antenna. The mounted antenna picture shows a CO-41A with homemade extended mast made from 6061 T6 0.120-inch wall thickness telescoping tubing from DX Engineering, stainless set screw collar machined for #3/8-24 threaded stainless whips, and the extra return element with a welded ferrule. Performance was about same as the 5/8-wave Dominator ground-plane but unmatched lightning immunity, as we thought the first antenna was taken out by lightning (turned out to be the antenna to feed-line N connector suddenly developed an open center pin connection!).

Conclusions:

This has been one of my most intense interests for years, most of which knowledge I have learned by trial and error by friends and myself. In fact, I just finished work on a 1/2 over 1/2 over 1/4-wave ground-plane that did not work as expected, but I learned most likely why, and in the process discovered a new way of increasing decoupling of a coaxial dipole to the mast. After a lot of work we ended up with just one very well decoupled 100% efficient dipole, and could not do a three-section collinear vertical with 3 dBd gain as planned. Here is a picture of the only partially working monstrosity 26 feet long that is now 21 feet.

6m-ant-pix/2.jpg

It turned out that the middle dipole could not be made to resonate and at the same time tune out reactance introduced to the top dipole, so the top portion of the middle dipole had to be shortened 10 inches from a 1/4 wave for the top dipole to have zero ohms reactance at resonance, and that made the middle dipole an extremely effective choke to make the mast invisible to the antenna (in my mind). The bottom 1/4 wave ground-plane was eliminated cause no RF was getting to it. So the antenna is now down to a true 0 dBd after all that work and expense, but has excellent lightning protection, because it is fed by a 13-inch loop between the skirt and mast with the coax inside the mast (Kreco shunt fed coaxial)!

Contact Information:

The author can be contacted at: jhaserick84 [ at ] comcast [ dot ] net.

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This article created on Sunday 29-Nov-2015



Article text and photographs © Copyright 2015 by John Haserick W1GPO.
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