COMROD 8-foot VHF Marine Band Antenna

VHF Marine Band radios, protocol, radio communication theory, practical advice; AIS; DSC; MMSI; EPIRB.
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COMROD 8-foot VHF Marine Band Antenna

Postby jimh » Mon Mar 19, 2018 9:39 am

An interesting VHF Marine Band antenna is made by COMROD in Norway. Their model AV60BI8 has these specifications:

  • Frequency range:
    156-159 MHz, VSWR < 1.5 : 1
    159-162 MHz, VSWR < 2.5 : 1
  • Nominal impedance = 50 ohm
  • Power rating: 100 Watts
  • Gain: 6 dB
  • Design:
    Collinear 5/8-wavelength phased brass elements. Radiating element completely enclosed in polyurethane foam within a fibreglass tube.
  • Length: 8-feet
  • Weight: 1.1 kg or 2.5-lbs
  • Wind rating: 55 m/s or 125-MPH
  • Mounting:
    The AV60BI8 is fitted with UNS 1-inch x14 female ferrules. Can be used with all standard mounting accessories. Integrated BNC female coaxial connector. All BI versions come with a tool to mount the BNC male coaxial connector inside the stainless steel ferrule.

The model AV60BI8 can be fitted to the standard 1 x 14 threaded base. The antenna is not supplied with an integral coaxial transmission line. The transmission line connects to the antenna via a BNC connector at the antenna base; the connector is concealed inside the metal base ferule. COMROD sells accessory transmission lines with a BNC connector at the antenna end and a FME connector the transceiver end; the include a PL-259/FME adaptor for mating to the usual SO-239 connector at the transceiver.

Unusual for marine antennas, COMROD supplies a vertical radiation pattern plot:

Vertical radiation pattern of COMROD AV60BI8
verticalRadiationPattern.jpg (31.08 KiB) Viewed 6892 times

This plot shows an extremely deep null (-40 dB) in the radiation pattern in the collinear directions above and below the antenna. This demonstrates why collinear vertical separation between an antenna and electronic gear or between one antenna from another is so much more effective than horizontal separation. The antenna has extremely reduced radiation in those directions.

Also unusual, COMROD supplies a VSWR graph. The VHF marine band for ship transmitters is confined to 156 to 157-MHz. In this region the VSWR will be extremely low, close to 1:1. AIS transmitters operate at 162-MHz, where this antenna will have a VSWR of about 2.5:1 or less when measured at the end of a 5-meter transmission line due to some line losses.

VSWR v. Frequency, COMROD AV60BI8 antenna
VSWR.jpg (11.77 KiB) Viewed 6892 times

Here are two views of the COMROD antenna installed on a hard top:

COMROD antenna mounted to hardtop. Photo credit Mark Sirof.
comrodAntennaOnHardtop.jpg (26.65 KiB) Viewed 6892 times

COMROD base with metal ferule. The transmission line connector is inside the base. Photo credit Mark Sirof.
extremeCloseUpCOMRODbase.jpg (27.1 KiB) Viewed 6861 times

This antenna is not cheap. The antenna has a MSRP of $397 but sells for around $297. An 5-meter transmission line with appropriate connectors sells for about $41. Because it is an 8-foot antenna, shipping could be expensive, perhaps $40. This puts the landed-cost of this antenna at about $378.

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Location: Southern California

Re: COMROD 8-foot VHF Marine Band Antenna

Postby msirof2001 » Tue Mar 20, 2018 11:55 am

I had a 1995 OUTRAGE 21 from October, 1994 - May, 2017. I kept it at a dry-stack where they covered-and-uncovered the boat, launched the boat, and stowed it. They replaced the VHF antenna a few times over the years, usually because the weight of the cover on the antenna at a wrong angle. When I ordered the Everglades 295cc, several new products in marine electronics had just been released. Because of my appetite for going long distances offshore, I wanted to go high-end in marine communications. Icom had just released the IC-M605 VHF Marine Transceiver and I have a lot of respect for the Icom products. The marine electronics installer (marine electrician) suggested a Comrod antenna. I had no prior knowledge or exposure to Comrod. He noted the rigidity and physical strength of the antenna, as well as excellent performance. He suggested the quality was commensurate with the other electronics I was purchasing. He noted the 125-MPH wind rating and strength should withstand the type of calamities that have broken my other antennas.

Comrod's website describes themselves as follows:
Comrod Communication Group is an international company with development, production, sales and marketing of antennas and antenna systems, support masts, power supplies, and battery chargers to the defense communication market. The Group also supplies antennas and antenna systems for commercial marine, as well as composite industrial products for the offshore and defense industry. Comrod Communication Group products are associated with innovative, high quality communication solutions and long-lasting products.

The Norwegian subsidiary, Comrod AS, is a supplier of antennas, antenna systems and power supplies, mainly for the defense market, but also for other maritime and commercial applications.

I am very happy with the radio receiver reception and audio clarity. In Southern California, VHF Channel 27 is dedicated to radio checks. When you hail for a radio check, a radio-check device thanks you for not using Channel-16 for that purpose, and then plays back your own transmission. It is extremely clear. I can hear things at greater distances with the new setup, as compared with the Shakespeare antenna on my old OUTRAGE 21. The Comrod is mounted about three-feet higher than on the Whaler. So height is a factor but I don't think the positive difference is solely because of a three-foot height increase. I believe it also has to do with the quality of the Comrod antenna.

Physically, the Comrod antenna has the feel similar to that of a polished ceramic tile, or of the actual hardtop, or like a Yeti Cooler. It is sturdy. I jokingly said in another article that I could probably use the Comrod antenna as a joust at a Medieval Faire. Maybe even as a pole-vault. All kidding aside, when it comes to safety on the ocean, and given the distances I travel, I'm not going to go the pennywise, pound-foolish route. I have had this setup less than a year but early indications are that it will service my needs for a very long time.
Current: 2017 Everglades 295cc, Previous1: 1995 Boston Whaler Outrage 21, Previous2: 1974 Sevylor Caravelle 3-man liferaft.

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Re: COMROD 8-foot VHF Marine Band Antenna

Postby jimh » Tue Mar 20, 2018 7:08 pm

ASIDE on your comment about VHF Channel-27 being used for unattended automated radio transmission replays:

The VHF Marine Band channel plan originally allocated Channel-27 for use by shore stations to provide ship-to-shore communication linked to landline telephones, called "Public Correspondence" channels. Several decades ago marine shore stations provided this sort of service to ships, but this has now largely been displaced by cellular telephones.

More than ten years ago the FCC auctioned-off ("sold") a large portion of the VHF Marine Band spectrum (Channels 24 to 28 and Channels 85 to 87) to a private company (MariTEL) who had a business plan to build an automated ship-to-shore-to-land-telephone network using the spectrum. That business plan failed--not a surprise with the rise of cellular radio telephone networks. In today's world, having a spectrum allocation is considered an asset, as many companies pay millions--or billions--of dollars to the FCC to obtain spectrum.

When the FCC allocates spectrum, the company acquiring the spectrum has ten years to demonstrate use of it and to show the service is in operation. Since there is just about no market for ship-to-shore telephone service, the spectrum owner (MariTel) began putting this VHF Marine Band spectrum to use in various ways. One use has been to partner with Tow Boat US to provide automatic radio-replay service on some of these allocated channels. While the service provided is somewhat useful to boaters, it is really not what was intended for these channels. By installing these automated replay servers on their otherwise unused channel allocations, the private company is demonstrating that they are "using" the frequency allocation, and thus can hold onto it. They have also converted some of their spectrum allocation to use for private yacht clubs. And they have leased out spectrum to non-Marine users, i.e., land mobile radio users, to companies like Motorola, Pacificorp, and Motorola/Riverside, in geographic area that are removed from ocean coastal, Great Lakes, or Western Rivers regions.

While you might think it is a nice service to get these replays on certain channels in certain regions, it is also a means for the recipient of a generous allocation of spectrum from the FCC to retain that allocation. In today's radio spectrum environment, owning any spectrum is considered a fungible asset that should be preserved and protected.

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Don McIntyre - MI
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Re: COMROD 8-foot VHF Marine Band Antenna

Postby Don McIntyre - MI » Thu Mar 22, 2018 7:19 pm

Jim - How does the performance compare to GAM setup?

And, I note that there's a required connection, instead of the antenna being supplied with a certain amount of cable. How much loss, if any would occur because of the connection?


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Re: COMROD 8-foot VHF Marine Band Antenna

Postby jimh » Thu Mar 29, 2018 8:58 am

I have no first-hand test data for the COMROD. Based on the manufacturer's description of stacked 5/8-wavelength elements the gain should be higher than a single half-wave element used in the GAM antenna. Note that COMRAD claim gain is 6-dB (no reference) and GAM claim gain is 3-dBd (dipole reference) or 5.1-dB (isotropic theoretical radiator reference). The reference antennas are probably in free space.

A wavelength at 158-MHz is 6.27-feet. Two 5/8th-wavelengths would be 10/8th-wavelength or 1.25-wavelength. That would suggest the antenna must be 1.25 × 6.27 = 7.8375-feet long. Since the antenna is described as being 8-feet long, it seems entirely possible that inside the radome there are two 5/8th-wavelength elements.

Loss in connectors in transmission lines is negligible assuming good connectors in good condition.