A question concerning VHF antennas...

[OneBoatman]

I carry an emergency VHF antenna. Mine cost less than 30 dollars about 25yrs ago. I have only used it once in more than 40 years of cruising. It is much more likely that the antenna on your masthead will be struck by lightening and be rendered kaput. This happened twice on my first sailboat. Each time there was no damage to the radio as the pots at the base of the antenna are as advertised "lightening proof", but they do get cooked. Of course you need to buy another or have bought a spare and go up the mast to replace it if the mast is still up. Remember your VHF is only good for short range communications perhaps up to 20-30 miles but if you are dismasted in the ocean chances are that even if you have an SSB antenna it also is dependent upon your rigging be standing and you would be similarly be out of action. I think it takes a fairly beefy bolt cutter to cut your rigging free should you be dismasted. When sailing either near or offshore you must be certain to reef early in deteriorating weather or get to a stormsail or bare poles before you are dismasted. You can always add more sail when conditions lighten up, but not if your sails are already shredded. The idea is to protect yourself, your sails, and your rig by making the right moves before you get slammed.

[continuouswave]

Originally Posted by toda

I have two separate antennas connected to my VHF...

More accurately, the two antennas are connected to a switch that selects one transmission to one of the antennas for connection to the transmitter according to how the switch is positioned and isolates the other transmission line from the transmitter with very high loss. (A well-designed switch might have 80 dB isolation; a poor switch might have only 30 dB isolation). With this arrangement only one antenna will be connected to the transmitter and will present a load to the antenna and will receive power directly from the transmitter. This is a very common arrangement, but differs from the initial question proposed for comments.

If the two antennas are close to each other, they will still affect each other due to coupling, and they will form a parasitic array with a new directional pattern that differs from the pattern for either antenna.

For advice on best practices for location of two VHF Marine Band vertical antennas on a boat, see

Mounting Two VHF Marine Band Antennas on a Small Boat - CONTINUOUSWAVE

The article discusses how to estimate the interaction between antennas using a well-known method.

ASIDE: the coaxial switch mentioned by TODA in his article claims to have an isolation of 115 dB between ports. This is a good example of the preposterous specifications the manufacturers of radio gear intended for use by recreational boaters employ. Even very expensive military grade coaxial switches typically do not claim more than 80 dB isolation. That a $33 switch has 115 dB isolation is from dreamland.

[edthec]

Originally Posted by chrisr

hi all

here is a question for anyone who knows about vhf antenna

like most boats, our vhf antenna is on the masthead. this raises the question what to do if the reason we are using the vhf is that we have been dismasted

we have a hand-held, but their power is limited

i'm wondering if it is feasible to have a second VHF antenna installed eg on the targa bar (which would probably survive any dismasting) ? what would be the affect of having both antennas connected to the VHF at the same time ?

thanks in advance

cheers,

Trying to use 2 antennas would cause a (technical term here) impedence mismatch affecting range. Might work for very short distances. It might be possible to buy a coax switch that allows you to connect a radio and two antennas so that you can switch between antennas. I know they're available for high frequency (HF) radios/antennas but not sure about their availability for VHF radios.

[ka4wja]

chrisr,

I saw this thread last week, but since Jim Cate and ozsailor gave you accurate and concise answers, within just minutes of your posting your question....I figured no need for me to wax on...

But, I wonder if Covid-19 caused the rest of you to drift things not just off topic, but to also pile on much false / misleading info??

So...maybe since I'm Covid Negative, I should add something useful... LOL


1) chris got his answers (thanks to Jim Cate and ozsailor), and that's great...all he needed to do is either run the cables out from the cabinetry to make changing cables easier, or use a hi-quality coax switch.

{and, of course, keeping an extra/spare VHF antenna below decks, in addition to your secondary antenna on a stern-rail, etc., while might seem like overkill, it is what I do....and, fyi, why not just make it the same as your primary antennas...except for saving a tiny bit of space below, there is no reason to buy/stow a small/compromise antenna, when a full-size 3' whip is damn easy-peasy to find room for below... }




2) I smiled when I saw slug's posting....unfortunately that pic is a pretty good look of the aftermath of a dismasting (albeit, probably one caused by a racing skipper pushing the rigging past its design limits, but in this discussion, it's a great pic showing why having a "spare" antenna below deck can be a good idea)....

Originally Posted by slug

It’s typical in a dismasting to loose everything on the stern of the boat.

This is the reason why I (and many others), in addition to our "back-up VHF antenna" / "AIS antenna" on the stern, or bimini, or arch, etc., I also keep a spare VHF antenna below decks....(of course, I also keep a 30' length of GTO-15 wire, to use as emergency HF-SSB antenna...yep, I'm a radio nut)....

Of course almost all dismastings are caused by rigging wire failure, when in heavy weather conditions...and/or after a 360* roll-over....so, while rare and usually can be avoided by good seamanship, an extra VHF antenna and a short length of coax are damn cheap, so why not just have 'em on-board....just in case.




3) And continuouswave gave a very nice clarification of the other complexities!

I especially enjoyed how he summed things up so nicely, with this paragraph!!

Originally Posted by continuouswave

In addition to the physical separation, the power and relative phase of that power delivered to each antenna affects the resulting radiation pattern. With random separation, random power division, and random phase relationship, the resulting antenna pattern will be equally random. To speculate on what may occur is meaningless.

{BTW, in addition to dozens of antennas I've designed/built over the past 5 decades, more than a few multi-element arrays, I have a 76 element VHF array, with four driven elements, that I designed/built for my moonbounce (EME) communications....which might sound impressive, but is fairly small by VHF EME standards....so, if you want to talk antennas sometime....well, I just don't have the time right now, though... }

Have a look:





4) Oh, on the subject of phased arrays / driven arrays....for those in US / Canada (and possibly in some other areas), you are certain to have actually seen a "phased array" antenna system, probably seen 100's of them over a few decades, but probably never knew it....

You see, if you separate two vertical antennas by a distance of approx. 1/2-wavelength, and drive them "in phase" (equal current to each, in phase), you end up with a wonderful "figure-8" pattern, broadside to the plane of the antennas, with a real gain over one single antenna of 3.8db...with fairly deep nulls off the end-fire sides (reducing interference from things off to the side)...

But, what if you don't have the room to space them 1/2-wavelength apart?
What if you only had enough room to space them approx. 1/4-wavelength apart?
Well, you'd still get some real gain of approx 1.1 to 1.3db, and have some shallow nulls off the end-fire sides....
So, for most applications 1.3db of gain over a single antenna, adding a second antenna and mount, and cable hardly seems like a useful endeavor....but...

But, what if there wasn't a good location to mount just one antenna?
Like a situation where mounting one single antenna on one side of a vehicle would distort the pattern?
And, mounting it in the center (where the pattern should be better), would cause this single antenna to be partially blocked by some part of the vehicle?
Well, in this case, having TWO antennas spaced only 1/4-wave apart, fed in phase....which should only provide about 1.1 to 1.3db of gain over a single antenna, will in actuality provide a fairly clean pattern (and reduced blockage-caused pattern distortions and loss of gain), which will show a real gain of as much as 3 to 6db, depending on the vehicle and exact antenna placements....
So, in this situation, using TWO antennas, fairly narrowly-spaced is actually a viable approach...

So, why in the world am I rambling on and on here?

Well, as I wrote above, those in the US and Canada have seen 100's of these "phased arrays" on the mirrors of trucks!


Those "18-wheeler's" (big trucks) on the highways, have their CB radio (27mhz) antennas, installed on their side mirror frames, giving them about 1/4-wave spacing....and they're fed in-phase and impedance-matched by feeding these 50-ohm antennas with a 1/4-wave of 75-ohm coax (of course using the cable's vel factor), transforming each to a 100-ohm load at the transmitter end of each cable, then feed in parallel giving the transmitter a 50-ohm load to feed...

And, since most of the original users (truck drivers using their 27mhz CB Radios) were on the US Interstate Highway System, traveling long distances in relatively straight lines / on relatively straight, long roads, having some gain broadside to the antennas (forward and behind the truck), as well as just improving the antenna pattern, all make for a rather elegant solution to a unique situation...

The reason I'm rambling on and on? To show you all that even with this simple easy-peasy situation of a CB radio on a truck, there's a lot that goes into this...
So, for my fellow sailors, please just use ONE antenna on your masthead (and a spare either below, or stern-rail mounted), and you'll be fine... LOL




4) Anyone that wishes some info on VHF Radiowave Propagation, antenna position/height, coax cable, etc....please have a look here:

VHF and AIS Radiowave Propagation and VHF and AIS Radio Range
https://www.cruisersforum.com/forums...ge-149499.html




As for much of the rest of the postings here...well, I just hope that Chris ignores most of them...



Hope I helped some?

Fair winds.

John

[Gypsyjon]

Since VHF is basically line of sight, power does not matter that much. The higher the antenna, the further away is the radio "horizon", but 5 watts will get you to almost any horizon.

[marti972]

Having done a lot of SOLAS radio inspects such as telegraph, telephone and satellite. Also, interested in most anything communication. I find it hard not to put in my two cents.



1. Learn how to make a VHF antenna out of coax and/or a few metals things you might have laying around. It's really simple. You can use Ham Radio 2 meter antennas as examples, just adjust the lengths for the marine frequencies.


2. If you find a bad VHF antenna somewhere, take it apart, you may be shocked how uncomplicated it is.



3. The antenna on top of your mast is a compromise. It is short because it might whack a bridge. A longer antenna would be better, but those bridges keep getting in the way. You can take some satisfaction from the fact that altitude is important.



4. I have seen a lot more VHF radios fail than antennas. This does not mean that radios might not fail more often. If the radio fails you will replace it. If the antenna fails you might not notice it. It doesn't necessary totally fail, it may radiate some giving you a false sense of security. Use an SWR meter! Standing Wave Ratio = the ratio of the power that is bouncing back, a ratio of more than 2 if concerning, much higher and you should start checking things. Google "SWR ratio" to get a table of how much power is bouncing back. Also, you could also just have a broken wire in your antenna connectors or your coax may be bad. The antenna might be just fine.



5. What is the plan, if you have to abandon ship? Your handheld is a good idea, if you have others nearby, but it has definite limits. Your VHF range just went down and your batteries may be old or discharged. I hope you have an EPIRB, a 406mhz EPIRB. If I am floating around out of VHF range, I want the whole world to know that I have an emergency and exactly where I'm at! That's why you want a 406mhz EPIRB. A long time ago, the only EPIRPs were 121.5/243mhz. If you still have one of these, note that the accuracy was limited and there are no satellites to hear you, anymore. It is not totally useless, but very limited.



6. I hope you put an extendible antenna on the hand held. It will give it a lot more punch, if extended of course.

[continuouswave]

Originally Posted by Stmurray62

Two antennae...

The plural of "antenna" is "antennas" unless you are submitting a formal thesis to a university in the U.K. in pursuit of a doctorate degree.

[chrisr]

hi to all

thanks for the latest comments. still like the idea of a coax splitter switch which will investigate further

my thinking concerning an emergency antenna goes like this : we never sail to windward, so if the mast goes, it will go forward. we are not going to do a 360 roll ! (180 maybe, but then it's a different game altogether), so imho there is a very good chance that an antenna mounted on the targa would survive a dismasting.

in such case, why not permanently mount the emergency antenna ready for use ? this could be via a switch or disconnect / reconnect cables

other issues such as epirbs etc are for another thread...

cheers,

[Rhumbline]

with a coax switch you can switch between antennas without connecting a cable. Cheap & easy. I use one for my HF ssb rigs. Keep the masthead as your main antenna due to its greater range. (line of sight for VHF)

[S/V Illusion]

Originally Posted by chrisr

hi to all

thanks for the latest comments. still like the idea of a coax splitter switch which will investigate further

my thinking concerning an emergency antenna goes like this : we never sail to windward, so if the mast goes, it will go forward. we are not going to do a 360 roll ! (180 maybe, but then it's a different game altogether), so imho there is a very good chance that an antenna mounted on the targa would survive a dismasting.

in such case, why not permanently mount the emergency antenna ready for use ? this could be via a switch or disconnect / reconnect cables

other issues such as epirbs etc are for another thread...

cheers,

A splitter isn’t what you want in the stated application. You want an antenna switch. A splitter is typically used only to switch between two radios using one antenna. Regardless, they are prone to fail without warning.

[W5PAD]

what would be the affect of having both antennas connected to the VHF at the same time ?

The simple answer is half the transmit power will go to both antennas assuming you did so by using a tee-connector. So in normal use you'd reduced your primary antenna's output power by half, e.g. to less than 12.5 watts (considering cable loss). If you are happy to lose half your power and do not want to manual 'switch' between antennas, use a tee. If you want full power, then use a RF switch or physically move the connectors as needed.

[continuouswave]

Originally Posted by chrisr

...like the idea of a coax splitter switch...

You'll have to invent that device, as they don't exist. Best of luck.

[Draughty]

Hi all,
My AIS transceiver recently started showing a red light which turned out to be an antenna fault.
I ran the proAIS2 software and for the first time I looked seriously at the diagnostics. It didn't tell me that the (masthead) antenna was faulty but it showed a VSWR of almost 10:1; a bit of research showed me that was BAD.
I hooked up to the pulpit mounted antenna (connector sitting there ready!), no more red light and VSWR 1.8:1 which seems to be acceptable.

The local coastguard said they heard me 5 x 5 using the dodgy antenna, which is only about 4 years old.

Some people who would like a VSWR meter might already have one in their AIS and like me not know it.

Rgds

[ka4wja]

Chris,
Details and specific recommendations below....
And, how much do I wish you'd have just taken Jim Cate's recommendation, a week ago...(but, I'm blaming Covid-19 for the plethora of weird responses here...LOL)

First some brief comments / corrections / quick advice...

Originally Posted by chrisr

hi to all

thanks for the latest comments. still like the idea of a coax splitter switch which will investigate further
What you're actually describing in your postings here, is a "switch"...

[Now, while there are "splitters", "switches", "relays", etc...and the exact nomenclature can be confusing to most layperson sailors, please be assured you're describing a "switch", and to keep from confusing yourself and everyone else, cease the use of the "splitter" portion of your question...
Btw, on a side note, most sailors now-a-days refer to RF-sensed transmit relays or RF-sensed transmit PIN diode switches, combined with a receive splitter....used to combine an AIS transponder onto the same antenna as their primary VHF....well, these are just now colloquially-called "splitters"....even though, there is a LOT more in them!
So, you see why there is confusion here in this thread about what you desire? ]


my thinking concerning an emergency antenna goes like this : we never sail to windward, so if the mast goes, it will go forward. we are not going to do a 360 roll ! (180 maybe, but then it's a different game altogether), so imho there is a very good chance that an antenna mounted on the targa would survive a dismasting.
I might have a differing look at this, but then I sail a monohull, and yes, I do sail to windward!!
But, in general, if you do wish to permanently mount a back-up VHF antenna (don't think we should call it an "emergency antenna?) on your stern arch / targa, then I'd recommend a 3' SS whip....as it won't shadow any solar panels (well not much, any way), and will not be easily damaged by an errant halyard, etc...


in such case, why not permanently mount the emergency antenna ready for use ? this could be via a switch or disconnect / reconnect cables
Yep, as mentioned earlier....it's either a switch, or access to connections that can be changed manually...


other issues such as epirbs etc are for another thread...
Yes...
Yes, please...let 'em have it....
(I mean I could wax on about DSC, the AMSA, WeFax, EPIRB's, etc. etc. etc....but that info is already covered and beaten to death...so, thought I'd just answer your questions...)

cheers,

So Chris, you've made your choice to "switch" between two antennas...and that's cool.

So, what you need is a high-quality coax switch....not a cheap "CB radio" switch, nor some crappy "marine store-version" of a cheap CB radio switch (both probably stamped out in a factory in China)....now while in your application, the exact isolation specs aren't too important, the actual "quality" of the switch, its thru-loss, and VSWR are!

So, I'm recommending a Daiwa CS-201 switch....and fyi, I own and use these myself (and have for a few decades), I have them at home and on-board....they are well made by a good Japanese company, and have excellent isolation specs, low thru-loss and excellent VSWR, all the way thru 600mhz, let alone VHF/150mhz....

The Daiwa CS-201 is reliable and solid, and provides you with a solid/certain switch position...yes, there are others out there, some well made but not quite up to the same specs as the Daiwa (like Alpha Delta), and still others that are unreliable and have poor quality control.

{now to be clear, they are not up to the quality of my Bird 74's / 72's....but the Birds cost > $1000 new, and a few hundred dollars, used...ouch!....but for your application the Daiwa is a good switch.}

It will handle 1000 watts of transmit power thru VHF/150mhz (and > 500 watts thru 600mhz), with min isolation of 60db thru 600mhz (and even higher at lower freqs), max VSWR 1.2:1 and max thru-loss of 0.12db thru 600mhz (and even better at lower frqes)...

http://www.daiwa-industry.co.jp/EN/products_hamsw.html

http://www.daiwa-industry.co.jp/EN/manual_cs201a.pdf



If you were in the US, I'd easily recommend ordering it from DX Engineering...at $33 USD, it's a bargain!!

https://www.dxengineering.com/parts/dwa-cs-201a


But, down under, you can find it here...
http://www.ttssystems.com.au/products/daiwa-antenna-switches/




FYI, you will also need a short coaxial jumper cable, to connect your VHF radio to the switch (the coax from each antenna connect to the switch, and an additional short piece of coax connects the switch to the radio)...

If you can fit the thicker cable, and not bend it too tight, I recommend a short jumper of RG-213....if you need a smaller cable, then use an RG-8x jumper...
https://www.dxengineering.com/parts/dxe-213udx003

https://www.dxengineering.com/parts/dxe-8xdx003

http://www.ttssystems.com.au/products/cables/



Of course, you will also need coax to run from the switch to your stern arch mounted VHF antenna....and if you're not experienced in assembling these cables/connectors, I recommend buying a pre-made cable assembly...and here again, I recommend RG-213 assemblies, unless you require a smaller cable...
https://www.dxengineering.com/search/brand/dx-engineering/product-line/dx-engineering-rg-213u-pl-259-low-loss-50-ohm-coax-cable-assemblies?autoview=SKU&sortby=Default&sortorder=A scending

http://www.ttssystems.com.au/products/cables/




As for antenna recommendations, I prefer Shakespeare...specifically the model 5215, which is a 3' SS whip antenna that can take a decent whacking with a halyard, or a bridge, etc...and survive for years....

http://shakespeare-ce.com/marine/product/5215-classic-vhf-squatty-body-antenna-copy/

BTW, if you wanted to use this back-up antenna as your primary AIS antenna, then buying the 5215-AIS version (which is just a 5215 tuned for the high-end of the VHF marine band)....
http://shakespeare-ce.com/marine/product/5215-ais-with-vhf-squatty-body-antenna/


I hope this helps...

Fair winds.

John


P.S. While most sailors' use of Marine VHF radio is within line-of-sight range....and yes, 5 watts on Marine VHF FM, at 65' off the water will typically have a greater range than 25 watts, 5' off the water....
Those that think VHF "is all line of sight", obviously haven't read the above mentioned thread....so, if you're at all curious, have a look...
VHF and AIS Radiowave Propagation and VHF and AIS Radio Range
https://www.cruisersforum.com/forums...ge-149499.html


P.P.S. While commercial / SOLAS installs may have (?) a greater radio failure than antenna / cable issues....on-board most private sailboats, and certainly on most offshore cruising boats, its very highly weighted towards cable/connector issues being #1, with the actual antenna being #2 on the list of failure modes VHF comms....(of course, this excludes episodes of lightning strikes)




P.P.P.S. Of course having an SWR meter is great....and yes, I have them (one spec'd for HF and one spec'd for VHF/UHF) permanently mounted at my Nav Station....but, unless you actually know what you're reading and how that reading down at the radio end of a lossy cable can be misleading, then they cannot be considered necessary...(at least in my opinion..)
{assume you had about 75' - 80' of RG-8x coax (which is typical on many mid-sized sailboats), this has a loss of about 3.7db (at best / when new), which means about 10.6 watts delivered to the top of the mast...

Now what if the antenna was totally gone? So that there was 100% reflected power?
Well, that 10.6 watts is now reflected back....but the loss of the coax going back down will deliver only 4.5 watts reflected back at the Nav Station...
So, what does the SWR meter read?

Hmmm...
You have 25 watts forward and 4.5 watts reflected, for an indicated VSWR of 2.4:1...
Now, of course a VSWR of 2.4:1 isn't great, but isn't all that bad either...
BUT...
But, that's the worst SWR that you can see on your SWR meter down at the Nav Station (assuming ~ 75-80' of RG-8x, without cabling or splice issues), even if you completely disconnect the antenna at the top of the mast!

Now, what if you just had a bad antenna / or a corroded or bad connection, or an antenna that was "bent" or cut-off due to a bridge, etc...and this might give you a 3:1 SWR at the antenna? (this 3:1 VSWR would reflect 1/4 of the original 10.6watts, or about 2.6 watts reflected back....which ends up being only 1.1 watts reflected down at the Nav Station....and that's a VSWR of 1.5:1...which is perfectly acceptable, and the casual sailor would never know that there is a problem with their masthead antenna...

NOW...what if you do want to learn how to determine if there really is a masthead antenna issue?
If all is okay, what should you see?
Well, let's just say we want to make sure that we never have an antenna feed-point SWR of greater than 2:1....this is because anything higher than that on a VHF Marine antenna, would mean there is an antenna problem...
So...
So, if we assume the same 75'-80' of RG-8x, and have the same 10.6 watts at the masthead being fed to the antenna, we'd have about 1.2 watts reflected...and that would be about 1/2 watt (~ 0.52 watts) down at the Nav Station....and an indicated VSWR of 1.3:1....
So, if you see an indicated VSWR of greater than 1.3:1 at the Nav Station, you either have a cable issue, a cable splice issue, or a damaged antenna / antenna issue....

Now, how many sailors are going to understand all of this....let alone how many will understand all the underlying calculations / engineering behind all of this??
Doubtful very many...
And, this is why recommending use of an SWR meter at the radio end of a VHF radio system isn't something that usually results in a "proof-of-performance" for a masthead VHF antenna...
Yes, it can show you that you might have an issue somewhere, most usually a connector issue at the radio end, or a cable splice issue....and sometimes a splitter or switch issue...but usually not pointing to masthead antenna issues....(of course, it CAN show this, and CAN point you there, but this requires some level of training / expertise that isn't typically found among my fellow sailors...

And, a final fyi...the "SWR meter" in ProAIS is only as good as your AIS transponder's measuring circuit, and even then I've not found the ProAIS software to actually show an accurate reading once above about 2:1....please understand that this is just anecdotal info, I have no engineering to show how accurately all transponders and ProAIS show VSWR, but the very short bursts of AIS transmissions are not conducive to simple / cheap forward and reflected power measuring...so, while a bad SWR reading on ProAIS is an indication of a problem, the exact reading can be inaccurate...

Whew...sorry about my ramblings....I just wanted to make sure that everyone understands the intricacies involved here...

take care and fair winds...

[Donald Bryden]

If you are open to a suggestion, I would suggest dropping the idea of a second VHF and substituting a Garmin Inreach Explorer. With it you use satellites to send messages. There is an SOS function where you tell a central location what your problem is and they inform the proper authorities. Once help is on scene, a handheld VHF is sufficient. Other features are tracking where friends can follow your sailing adventures, you can get weather forecasts, trip information such as speed, time underway, and a odometer. The cost for a unit is less than $400. There is a monthly fee of less than $40 and the membership can be turned on and off as needed. Meaning no fee during layup, etc. We find ours very useful.