A Captivating Saga of VHF Antennas

VHF Marine Band radios, protocol, radio communication theory, practical advice; AIS; DSC; MMSI; EPIRB.
jimh
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A Captivating Saga of VHF Antennas

Postby jimh » Sun Aug 09, 2020 10:57 am

As I mentioned recently, I had a failure of the antenna extension mast on my boat's VHF Marine Band radio antenna installation. Repairing this failure has led me to an entirely new approach to antenna installation. I will explain in a multi-part narrative.

The 13-year-old antenna installation consisted of a GAM ELECTRONICS SS-2 antenna and ADAP-II base adaptor, mounting atop a four-foot extension mast Shakespeare model 468. In trying to disassemble these three components, there was quite a problem. The transmission line between antenna and radio was captivated in place in two positions: the feed line ran down the inside of the extension mast and exited at a hole in the base; and the feed line passed through a bulkhead fitting on the cabin topside. The diameter of both these passages was only large enough for the coaxial cable itself to pass through. In radio work, we say the transmission line was captivated in place by these two small passages. In order to extract the coaxial cable from the extension mast, I had to cut off the (very recently installed) connector at the radio end of the cable, pull the cable out of the cabin feed-through, and then remove the ADAP-II adaptor from the extension mast and extract the cable from the extension mast.

The connector at the radio end of the cable was a FME (For Mobile Equiment) female cable-end straightconnector that I had just installed a few months ago. (More information about FME connectors is available in a prior article in this forum.) Although the cost of this connector is very low--only about $3--and I had more of them on-hand, it was irksome to have to cut-off a perfectly good connector in order to perform a routine repair to the extension mast. Also, the connector used a solder-on center conductor and a crimp-on shield conductor. I had the proper crimp tool on hand, but the soldering iron I had available had a tip that was much too large to work on the very small center pin contact. Also, I decided I was not going to install another connector just to re-mount the GAM SS-2 antenna without the mast, as later I would have to cut-off that connector again to put an extension mast back into the system.

A further problem encountered was the need to unthread the ADAP-II adaptor from the upper end of the extension mast. The two parts fit together using the standard 1-inch x 14 pipe threads. The lower end of the transmission line passed through a hole at the bottom of the extension mast. Unthreading the ADAP-II adaptor from the extension mast would require about 14-turns, with each turn imparting a twist to the coaxial transmission line. I did not want to impart 14-revolutions of twist into the transmission line, so it became necessary to first remove the extension mast from the steel antenna base, a typical ratchet mount.

The point of failure of the extension mast was actually at the base in the section that threads onto the steel mounting base. The base of the extension mast is made of a soft material, some sort of plastic that might have originally been PVC. But 13-years of exposure to sunlight and weather seemed to have softened the plastic material somewhat. The friction between the threads of the plastic base and the steel mount was very high. Removal by hand was impossible. A large slip-joint pliers was needed to gain enough purchase and leverage to unthread the mast. Of course, pressing the pliers onto the plastic base with enough force to be able to rotate the base to unthread it created further friction. The room available for the pliers was also limited by the location of the mount. I had to unthread the mount about 1/8th-turn at a time. The 14-revolutions took about 100 or more 1/8th-turns on the base--a very tedious process. All the time the extension mast was being rotated, the coaxial cable feed line had to be carefully allowed to wind up in a spool around the extension mast, as it was still captivated at the base of the mast.

Aggravating the unthreading of the base from the steel mount was a partial fracture in the plastic base. This caused the upper part and lower part of the base to be threaded at different points on the steel threads of the base, further increasing the friction between the plastic and steel parts. Getting the extension mast unthreaded while not damaging the coaxial cable transmission line captivated in the base required careful use of tools.

Of course, eventually I got the extension mast off the mount. The next step was to unthread the ADAP-II mount from the top of the extension mast. Again, about 14-revolutions were needed. And again the transmission line would be rotated or twisted with each revolution. I pushed the transmission line back through the exit hole at the base of the extension mast to free it from being captivated there. I took all the twists out of the cable, and laid it out in a straight line. Then I began to unthread the ADAP-II adaptor from the top of the extension mast. Each rotation of the adaptor during unthreading imparted a twist to the coaxial cable. After about two rotations I stopped and untwisted the transmission line two turns. Then back to unthreading the adpator from the extension mast two more turns. This was repeated until the ADAP-II adaptor was completely unthreaded from the mast.

With the broken extension mast removed, I began to re-assess the antenna system installation. Repair of the broken extension mast had required too much disassembly of the antenna system to effect a repair. I determined that I would re-design the antenna system so that in the future the transmission line could be run from antenna to radio via passages that would be large enough to allow the FME female cable-end connector to pass. The FME connector will fit through a hole of 3/8-inch diameter. In this way the antenna and its transmission line could be removed without have to cut off the connector.

[More installments to come.]

jimh
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Re: A Captivating Saga of VHF Antennas

Postby jimh » Mon Aug 10, 2020 9:35 am

As seen in the sketch below (Figure 1), in order to remove the damaged extension mast from the antenna mounting, the FME connector had to be cut off so that the bare cable could be pulled out of the boat and eventually out of the extension mast, after the mast was disconnected from the mount.

cablePath.png
Fig. 1. The path of the antenna transmission line cable passed through two captivating locations, the extension mast and the cabin bulkhead. Although the FME connector needs only a 3/8-inch diameter hole to pass through, the holes in the extension mast and the bulkhead fitting were too small.
cablePath.png (26.98 KiB) Viewed 659 times

jimh
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Re: A Captivating Saga of VHF Antennas

Postby jimh » Mon Aug 10, 2020 1:31 pm

The first component to change to remove a cable captivation problem was the bulkhead pass-through fitting. A replacement fitting would have to permit the cable and connector to pass through. This lead me to investigate the size of various coaxial cables and their connectors.

Marine antennas generally use only two types of coaxial cable for their transmission lines, usually RG-58 or RG-8X. The RG-58 cable is made in several varieties but all tend to have a outside diameter of 0.195-inch. They will easily pass through a 1/4-inch diameter hole. The RG-8X cable is made with a larger outside diameter, nominally 0.242-inch. It should also fit through a 0.250-inch diameter (1/4-inch) hole, although perhaps more snugly. A great reference table of cable dimensions is found at RFCAFE.COM.

The connector will always have a larger diameter than the cable. I am converting to use of FME connectors, with a female cable-end connector on the transmission line. This creates the smallest diameter connector. The hex-shaped retaining collar is specified to be 8-mm across the flats or 0.315-inches. Finding the smallest hole that this will pass through requires some geometric calculations. By my calculation the hex-shaped collar needs a hole diameter of 9.24-mm diameter or 0.364-inches. The FME female cable-end straight connector should fit through a hole of 0.375-inch (3/8th-inch).

A UHF-series cable-end straight plug, better known as a PL-259, will have a maximum diameter of 18-mm or 0.71-inches. A 0.75-inch diameter or 3/4-inch hole will allow this connector to be passed through.

I chose a product from BLUE SEA SYSTEMS, which they call a "Cable Clam." This product is made in several versions, with a different range of connector and cable diameters accomodated. Next installment will detail the "Cable Clam."

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Re: A Captivating Saga of VHF Antennas

Postby jimh » Wed Aug 12, 2020 9:50 am

BLUE SEA SYSTEMS has a line of fittings for passing a cable through a bulkhead and maintaining a weather-tight seal. They call their product a "Cable Clam." They offer these in a range of sizes. There are two controlling dimensions, the size of the circular backplate (deck ring) that holds the assembly to the bulkhead, and the size of the hole in that backplate that will limit the diameter of the hole in the bulkhead. The smallest model is the 1001. The circular assembly has a diameter of 1.85-inches, and there is a center hole with a diameter of 0.79-inch. The center hole at 0.79 inch will be large enough to permit a PL-259 connector to pass through. There is a compression ring with a somewhat larger center hole, and the cable and connector must be passed through this hole, as well.

blueSeaSystemCableClam1001.jpg
FIg. 2. Blue Sea System Cable Clam model 1001. This product retails for about $18. It includes the four machine screws and four self-tapping screws shown.
blueSeaSystemCableClam1001.jpg (66.14 KiB) Viewed 601 times


The black circular plug seen on the upper right in Figure 2 is a slightly tapered conical section rubber seal for the cable to pass through. During installation, the installer must drill a center hole in this plug that is appropriate for the diameter of the cable. Then the installer cuts a slit in the plug from the center hole to the outer rim, so that the seal may be slipped over the cable.

In assembly of the cable clam, the cable and connector are first passed through the compression ring, then though the deck ring, and though the hole in the bulkhead. Then the seal is slipped onto the cable, and the compression ring tightened down onto the deck ring to complete the seal and installation.

For my installation with an FME female cable-end connector on RG-58C/U transmission line, only a 3/8-inch hole will be necessary in the bulkhead. The original 1/4-inch hole in the bulkhead will be enlarged to 3/8-inch, and the 1.8-inch diameter deck ring will nicely cover the old mounting holes for the original and much smaller bulkhead fitting.

Blue Sea Systems also refers to the model 1001 as having a maximum cable size of 0.68-inch. I believe this refers to the maximum diameter of a hole that can be made in the seal plug by drilling. For drilling the center hole in the seal, I plan to temporarily install the seal in place under the compression ring, and drill the hole in the seal while it is on the boat bulkhead. I have not yet installed the cable clam. I will remark on the actual installation process in a future update.

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Re: A Captivating Saga of VHF Antennas

Postby Jefecinco » Thu Aug 13, 2020 9:54 am

I have used the Cable Clam and found it to work well and it was very durable. Ten years after installation it remained watertight and attractive.
Butch

jimh
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Re: A Captivating Saga of VHF Antennas

Postby jimh » Thu Aug 13, 2020 11:36 am

BUTCH--thanks for the first-hand report on the Blue Sea Systems Cable Clam.

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Re: A Captivating Saga of VHF Antennas

Postby jimh » Thu Aug 13, 2020 11:41 am

I made a short table to compare several coaxial transmission lines used with marine antennas.

Comparison of small-diameter flexible coaxial cables for 150-MHz

Type        Loss 100-ft  OD
150-MHz Inches
LMR-240-UF -3.6 dB 0.240 Times Wire https://www.timesmicrowave.com/DataShee ... 240-UF.pdf
RG-58C/U -6.8 dB 0.195 Belden 8262 https://catalog.belden.com/index.cfm?event=pd&p=PF_8262
RG-8X -4.1 dB 0.242 Belden 9258 https://catalog.belden.com/index.cfm?event=pd&p=PF_9258

Note loss is for 100-feet. Scaled to 10-feet loss is

Type Loss 10-ft
150-MHz
LMR-240-UF -0.36 dB
RG-58C/U -0.68 dB
RG-8X -0.41 dB

Loss compared to RG-58C/U for ten-foot length of transmission line:
Using RG-8X only saves 0.27 dB
Using LMR-250-UF saves 0.32 dB


For transmission lines of about 10 to 15-feet, using a larger and more expensive coaxial cable really does not improve the transmission line loss significantly.

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Re: A Captivating Saga of VHF Antennas

Postby jimh » Wed Aug 26, 2020 3:03 pm

Before I could replace the extension mast and fix the captivated cable problems, I need to get something working for the boat radio. I installed the GAM SS-2 antenna on its ADAP-II mount and put it directly on the base hinged mount, without an extension mast. I had to run the transmission line through a small hole in the base mount. This hole was located on the inboard side of the mount, so it created a tight bend in the transmission line because of the close clearance to the cabin superstructure. And the antenna was mounted very low, so the radiating element was right next to all the helm electronics. Getting the antenna radiator away from the instrument panel was one of the original objectives for using the GAM SS-2 with an elevated mounting on an extension mast. I knew this mount was only going to be temporary.

gamSS2_noExtensionMast.jpg
Fig. 3. The GAM SS-2 antenna on the ADAP-II mount with feedline exiting at the bottom, running through the hinge mount and exiting on the inboard side.
gamSS2_noExtensionMast.jpg (102.11 KiB) Viewed 441 times


Feeding the transmission line cable through the hinge mount base hole was impossible with the mount in its normal orientation. Even with the mount re-positioned to let gravity help, getting the cable through it was tedious.

hingeBaseCableExit.jpg
Fig. 4. The hinge base cable exit hole was oriented inboard in the normal position of the mount. Here I have loosened the mount to make it much easier to thread the cable through. I vowed never to do this again.
hingeBaseCableExit.jpg (60.67 KiB) Viewed 431 times


A further problem was the transmission line had to be run through both the mount and the bulkhead without a connector. I did not feel like putting on a connector that I knew I would be cutting off in a week or two, so I made a very temporary splice in the coaxial transmission line.

transmissionLineSplice.jpg
Fig. 5. A very temporary splice in the transmission line; no solder was used.
transmissionLineSplice.jpg (41.2 KiB) Viewed 441 times


The boat VHF Marine Band radio was quite pleased to transmit and receive through that rather crude splice for the couple of hours we had the boat underway and used the temporary antenna. A splice like the one shown is certainly not suitable for a long-term solution, but it worked well as a temporary fix.

Mounting the antenna this low convinced me that using an extension mast was mandatory. I was also reminded how awkward it was to have to use the screw-on antenna adaptor and wind up 14 or more revolutions onto the transmission line in the process. I set about to eliminate both these problems.

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Re: A Captivating Saga of VHF Antennas

Postby jimh » Wed Aug 26, 2020 3:32 pm

Another week went by before I was able to install the BLUE SEA SYSTEMS Cable Clam. (When you are almost retired, it is amazing how fast a day can go by with not much getting done.)

The first step was to remove the existing cable pass-through fitting. Its three #6 mounting screws came out easily, but the fitting was very firmly adhered to the cabin superstructure gel coat. I put pliers on it and tried to give it a twist--nothing doing with that method. Finally I got a chisel and hammer. With the steel chisel blade right at the joint line between metal and gel coat, I gave a whack with the hammer. That did it; the fitting was loose.

My guess is the originally installer did not use a sealant but perhaps used a marine sealer-adhesive product like 3M4200 or (God forbid) 3M5200. Those products are very strong adhesives. I made a plastic scraper out of throwaway plastic knife from some take-out food, and used this scraper to remove the remaining sealant-adhesive.

The sealant had made a very good weather seal because underneath was pure unweathered 28-year-old Boston Whaler Desert Tan gelcoat, which stood in contrast to the surrounding gel coat surfaces, which--admittedly--were quite neglected.

originalGelCoat.jpg
Fig. 6. The original gel coat color now seeing sunlight for the first time in perhaps in 30-years.
originalGelCoat.jpg (26.77 KiB) Viewed 437 times


This color mismatch was so stunning that I had to immediate fix it. I cleaned off the just-developing mildew and dirt, and I wet-sanded with 600-grit to get rid of the oxidation of the gel coat surface. As usually happens, the Boston Whaler gel coat returned to new appearance--at least while wet.

wetSandedGelCoat.jpg
Fig. 7. The gel coat after cleaning and wet sanding.
wetSandedGelCoat.jpg (16.18 KiB) Viewed 437 times


I wanted to reuse the existing center hole and also cover the existing but now unused original three screw holes. The existing center hole was 0.25-inch diameter. I enlarged the center hole of 0.375-inch, then to help center the new base ring of the Cable Clam, I used a 0.375-inch drill stuck into the existing hole and turned a small socket into a flange, making my own alignment fixture.

alignmentFixture.jpg
Fig. 8. An alignment fixture made from a drill bit, a socket, and some blue tape.
alignmentFixture.jpg (31.47 KiB) Viewed 435 times


After carefully marking the holes onto the blue tape, I removed the base ring and drilled the first hole. This hole needs to be right on the money--I did my best. As always when drilling into 28-year-old brittle gel coat for a screw fastener hole, the top layer of the boat hull, the gel coat layer, must be enlarged in diameter so the screw fastener is not trying to cut threads into it. If the gel coat is not enlarged it will almost always crack when a screw is forced through it.

I reinstalled the deck ring and one mounting screw to fix it in position. To drill the remaining three holes I used a drill with the same diameter as the mounting holes on the deck ring to mark the center of the hole. Then a smaller drill made the through-hole guided by the indentation of the larger hole. Then I carefully enlarged the gel coat at the top of the screw hole, and also slightly enlarged the hole itself so the self-tapping screw threads were cutting away just the a minimal amount of larger hole diameter.

At this point, I seemed to stop taking pictures of the process. I will narrate the rest.

I mounted the deck ring with its gasket onto the cabin superstructure. I did not use any sealant, as the gasket seems sufficient. Next I installed the undrilled rubber seal into the compression ring, and installed the compression ring onto the deck ring. I did not tighten the four machine screws completely, but just enough to hold the seal in place so I could drill a center hole in it.

The undrilled rubber seal is over a half-inch tall, and you must drill your own hole in it, appropriate for the feed line that will be passing through. The instructions counsel you to use a sharp drill and lots of water. The rubber seal is soft, so the drill tends to thread itself through the rubber rather than cut away a clean hole. Eventually after several water-cooled passes of a 0.250-inch drill through the rubber seal, I was satisfied the hole was as clean as I could make it, although it was far from perfect.

The final step in preparing the rubber seal is to cut a slice through it radially from the outer edge to the center hole, so you can fit it over the transmission line cable. A really sharp Exacto knife allowed the cut be to be very easy to make.

The Cable Clam was now installed. It just needed an antenna transmission line to be passed through the compression ring, through the seal, through the center hole in the deck ring, through the superstructure, and on inside the boat cabin.

I will describe the new antenna mounting and transmission line system in the next update.

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Re: A Captivating Saga of VHF Antennas

Postby jimh » Wed Aug 26, 2020 8:11 pm

I was initially intending to re-mount the GAM SS-2 antenna and its ADAP-II mount on a new four-foot extension mast. The antenna transmission line, which is part of the ADAP-II assembly, exits in the center and must come down the center of the extension mast. In order to not return to the same situation as before in which the feedline passed through a small hole at the bottom of the extension mast, I needed to find an extension mast that had at least a 0.375-inch-diameter hole at the base.

The most reasonably-priced four-foot extension mast I could find was a Shakespeare 4008 series mast, the 4008-4. I called Shakespeare technical support to inquire about the dimensions of the hole at the base of the 4008 mast and what its diameter would be. I got a nice woman in customer support on the telephone, and she assured me that the hole was 0.375-inch diameter. But I was not convinced. I called back later and got a different woman on the line at customer support. I asked her if there was a drawing of the mast that showed the hole and its dimensions. She emailed me a PDF file that was the installation instructions, and it did have a drawing of the extension mast. But the dimensions of the hole were not clearly shown.

Because the 4008-4 extension mast is about $90, I was not ready to commit to spending $90 until I was convinced it would work with my plan to eliminate the transmission line from being captivated in the extension mast. I did not want to order a $90 mast to find out it would not have the right hole dimensions. So I set aside the project to re-mount the GAM SS-2 on an extension mast until I could verify that the Shakespeare 4008-4 extension mast would have an appropriate opening at the base to permit the transmission line with the FME connector installed to be able to exit.

Now the focus on the antenna transmission line cable with connector installed being able to be passed through the extension mast and out the side at the base could be completely eliminated if I had a better hinge mount. Modern marine antenna hinge mounts now tend to have a hole in their threaded bases with about a 0.375-inch diameter, enough to let the FME cable-end female straight connector to pass through. And I actually already had one of these nice bases on hand.

With the heavy-duty hinge base with cable access hole already on hand, I tested the clearance of the hole by inserting a cable with connector. For this I used the short cable and FME cable I had to cut off from the original disassembly of the antenna. I never throw anything away, as you never know when it might come in handy. The cut off cable and connector provided a real test of any access hole to see if you could pass the cable and connector through the hole. I found that the cable and connector would fit through the opening.

hingeMountBaseWithCableHole.jpg
Fig. 9. A heavy-duty hinge mount base with a cable access hole, with an FME female cable-end straight connector shown passing through.
hingeMountBaseWithCableHole.jpg (133.34 KiB) Viewed 420 times


I found that passing the cable and connector up through the hole was very easy, but passing it down through the hole was much harder. This was due to the limited bend radius available for the cable to bend inside the mount. But you could fish-out the connector coming downward through the base mount if you used a small tool to help guide it.

This new mount would then solve the problem of the extension mast having or not having a proper hole at the base; the transmission line would just exit the extension mast at the base through the center of its female threaded mount.

Why didn't I just remove the original antenna hinge base from the boat and replace it with this new and better one? The reason is the difficulty I had getting off the old cable pass-through. The area of the old cable pass-through base which was adhered to the cabin superstructure was relatively small, and that adhesive was a real bugger to break loose. The antenna base has a large metal plate about 3 x 4-inches. If that was adhered with the same sealant, I was very concerned if I could even get it loose. I did not want to hit it with a hammer because, as structural parts, the metal base is many times stronger than the cabin laminate, and I did not want to break off a piece of the cabin laminate. I also had some history with the strength of the adhesive. (See another old article about trying to remove a through-hull SONAR transducer.)

Way back years ago when I was installing the GAM SS-2 antenna I decided that the hinge base needed to have a backing plate in the inboard side of the cabin laminate. I made metal backing plate. I removed all four mounting screws from the base, and discovered that the base did not budge one millimeter with the screws removed. It was so firmly adhered to the cabin laminate that it supported the entire antenna base, the extension mast, and the antenna without any mounting screws in place.

I did not want to turn the current project into a week-long nightmare project, so I decided to save the better hinge base mount for another antenna installation on another day. I was going to have to live with the existing base mount whose cable hole was too small for the FME connector to pass. A different antenna solution was going to be necessary. I will explain that choice in the next installment.

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Re: A Captivating Saga of VHF Antennas

Postby jimh » Thu Aug 27, 2020 4:54 pm

With the old antenna and extension mast set aside, I turned to a new antenna project: installing a MORAD VHF-156 HD antenna. This new antenna system was going to be built so there were no threaded connections between the antenna and the extension mast, and there would be plenty of room for the transmission line and connector to pass through the extension mast, the base mount, and the cabin bulkhead.

The new antenna system consists of these MORAD components:


The mechanical assembly of this system has only one threaded component: the M87 base threads onto the standard 1 x14 threaded ratchet hinge mount. You can install the M87 based onto the mount, and then build the rest of the antenna without having to screw-on any other component. The base is 4-inches long, so it adds some height to the system

The V2 extension is 2-feet long. It is just a nicely fabricated piece of aluminum tubing with an excellent white paint finish. The bottom end fits into a socket on the M87 mount. Three set screws are tightened to hold the V2 mast
Attachments
antennaConnector.jpg
Fig. 10. The recessed UHF-series antenna connector on the MORAD VHF-156HD antenna with transmission line connected.
antennaConnector.jpg (125.78 KiB) Viewed 387 times
MORAD_VHF-156HD.jpg
Fig. 12. The MORAD VHF-156HD antenna installed, showing overall height in operating position.
MORAD_VHF-156HD.jpg (89.03 KiB) Viewed 396 times
moradMountExtensionMast.jpg
Fig. 11. The MORAD M87 mount adaptor with V2 extension mast. The RG-8X transmission line exits via the generously large hole in the mount adaptor and passes through the Blue Sea Systems Cable Clam.
moradMountExtensionMast.jpg (58.51 KiB) Viewed 396 times

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Re: A Captivating Saga of VHF Antennas

Postby jimh » Sat Aug 29, 2020 8:55 am

Following up on the Cable Clam: there are several different size ranges. For small coaxial cables the smallest size is best suited.

Blue Sea Systems offers the Cable Clam in two versions; they are the same except for the inclusion of a stainless steel dress cap on the second version. The dress cap version costs $8 more, but it comes with two rubber seal plugs that are pre-drilled.

The Cable Clam that I used is part 1001 and MSRP is $21.99. The dress cap version is part 1001100 and MRSP is $29.99. The two rubber seal plugs included with the dress cap version are pre-drilled with different hole sizes; 0.165-inch and 0.220-inch.

Comparing the hole size with standard coaxial cable transmission lines, I don't find an exact match for either:
  • the 0.165-inch hole is smaller than RG-58C/U cable at 0.195-inch, 0.030-inch undersize;
  • the 0.220-inch hole is smaller that RG-8X cable at 0.242-inch, 0.022-inch undersize.
The slight undersize is probably intentional to assure a good seal.

With a pre-drilled hole adjusting the hole size with a drill should be easy. I also would presume the pre-drilled hole is right on exact center. Drilling a new hole in a black seal plug and getting it exactly on center is not always a guarantee.

I suspect the seal plugs might really be from chemistry supplies. Compare at

https://www.sciencecompany.com/Black-Ru ... Chart.aspx

Here are links to the Blue Sea Systems website for both products:

Dress Cap Version
https://www.bluesea.com/products/100110 ... Cap_0.68in

Plain Nylon Cap version
https://www.bluesea.com/products/1001/CableClam_0.68in

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Re: A Captivating Saga of VHF Antennas

Postby jimh » Sat Aug 29, 2020 10:59 am

On extension masts: I discovered that I could make my own extension mast using components from MORAD. The mast would be composed of three parts:
  • a base adaptor, M87 ($40); this threads onto the typical ratchet mount, and mates to a 1-inch stanchion
  • a 1-inch stanchion, available in two and five-foot lengths: V2 ($30) and V5 ($50)
  • a top adaptor, the M89 ($30), which mounts to the stanchion and then provides a threaded 1 x 14 coupling for antennas with that type base to mount to
The extension mast created would be all metal, and would have a generous opening at the base for the antenna transmission line cable and connector to exit. The assembly of the threaded components could be done before passing the transmission line through the mast, so the tedium of having to turn the coaxial cable in synchronism with the threaded of the mast is eliminated. The total cost is not crazy: A two-foot mast is $102, which is a bit expensive, but a five-foot mast is just $30 more at $122. Those are MSRP prices from MORAD. MORAD is not sold by the usual big marine distributors, so street prices are usually only at a small discount (maybe four-percent) from MSRP.

In contrast the Shakespeare 4008-4 four-foot fiberglass extension mast retails for $195. That is much more expensive, but typically the Shakespeare products are sold at substantial discounts by big marine distributors. The street price for a 4008-4 may be around $85.

The advantage of the MORAD make-your-own extension mast approach:
  • all metal thus excellent strength and durability
  • excellent connector clearance for transmission
  • wide range of heights available by cutting metal stanchion to custom length

Positioning the antenna transmission line inside the metal extension mast may also help avoid antenna currents from flowing on the transmission line. Antennas currents flowing on the transmission line often cause problems with pattern distortion, interference to other devices whose cables are near the transmission line, and may permit ingress of noise into the antenna and thus into the receiver.

In the event of damage, the extension mast described above made from three separate components would be unlikely to need complete replacement. The stanchion tube might be the only component that would need replacement.