A question concerning VHF antennas...

[Jammer]

Problems with the cabling or antenna at the top of the mast are far more common than dismastings. Corrosion is the usual culprit. Good reason to have a spare that is both convenient and effective.

Originally Posted by Mark Thurlow

Interesting thought about h/h to main antenna: I was told that would not work and that I was better off with the H/h stubby. Thoughts?

I don't agree. Most handhelds do fine with an external antenna. If in doubt, try it. It is wise to use lightweight, flexible coax for the last few feet leading to the handheld if this is your setup for ease of use and to reduce the stress on the handheld's antenna connector. RG-316 or similar.

[Jammer]

Originally Posted by jmschmidt

A short metal masthead antenna, due to the narrow beam that it transmits, cannot connect with a receiving antenna on shore or on another vessel if the sending vessel is rocking and rolling in heavy seas.

Respectfully, this simply isn't true. You can run a test by getting signal reports from another boat while you are heeled over and see for yourself if you don't believe me.


The short (~18") stainless steel whips, and the somewhat longer sleeve dipoles (Shakespeare 5400 etc), do have difficult-to-model lobes at the top of a mast in the real world, but these differences in gain are insignificant in the VHF world, and are certainly no worse than those that occur with a lesser antenna mounted lower. The height, with the more distant radio horizon, is what matters most.


Top-of-mast antenna installations that perform badly are caused by deficiencies in cabling, connectors, installation, or the antenna itself. VNAs are available on ebay for under $100 that will find these problems and accurately assess the performance of the feedline system.

[Brian.D]

Back to the original question of VHF antennas and demasting. Well, I decided to build my own. It is a half-wave antenna oriented in a vertical position so that the radiation pattern is the same regardless of the antenna being swung around. The antenna is made from PVC tubing and copper adhesive tape. The large black roll is a common mode choke. This antenna is used in my home for regular VHF communications, but it goes on the boat when I go out. It can be strapped to a boat pole and raised to the full extent of the pole.

These are easy to make and do not require any major equipment. On the other side of the VHF antenna is a UHF antenna at 440 MHz.

Just thought I would throw this into the mix.

[Lake-Effect]

Originally Posted by Brian.D

Back to the original question of VHF antennas and demasting. Well, I decided to build my own. It is a half-wave antenna oriented in a vertical position so that the radiation pattern is the same regardless of the antenna being swung around. The antenna is made from PVC tubing and copper adhesive tape. The large black roll is a common mode choke. This antenna is used in my home for regular VHF communications, but it goes on the boat when I go out. It can be strapped to a boat pole and raised to the full extent of the pole.

Looks durable. Have you measured its SWR at CH16? Why the common-mode choke?

[Brian.D]

Yes, used an Antenna Analyzer and it was about 1.3:1 +/-. This is because this antenna is also used on the 2M ham bands so the lower SWR is located in that band.

The common mode choke is for my higher wattage VHF use on my FT-857D. I use the antenna on 50 watts at times. So I want to keep the CM out of the radio. Not needed for Marine VHF.

added - I've used this antenna to reach the Catalina Island repeater from my home in Oceanside. The repeater is about 60+ miles line of site, sits on a peak of around 1500+ feet and my home is around 250+ feet above sea level. Based on calculations, the repeater antenna is just barely above the horizon.

[KP44]

Originally Posted by chrisr

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

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

thanks in advance

cheers,

VHF radios are line of sight. So it goes to the horizon. Just 1 watt is usually enough and handhelds on high power broadcast about 5 watts.

Always use only one antenna at a time.

A radio has only one antenna, no matter how many parts or pieces. Putting up two radiators (separate antennas connected by coax) will create all sorts of unwanted resonances and probably out-of-phase signals. (Distortion) But the worst part is the standing wave in your coax will probably be high because your antenna will no longer match your radio's impedance.

A standing wave is like an ocean wave in a current. If the wave is moving at the same speed as the current but in the opposite direction, a standing wave builds and stays there. This is what can happen when ocean waves get to a river bar during an outgoing tide. Or a wind is creating waves against the flow of the Gulf Stream.
Standing waves occur in the coax between radio and antenna when there is enough of a mismatch of impedance. When the radio senses the standing wave is too big, the radio shuts down its transmit power to keep from damaging the radio's transmit circuits. That is why a mismatched antenna gives such low output. And using high power does not help because most of that power becomes what could be called interference.
(An SW meter is a standing wave meter.)
I hope this long, amateurish explanation is helpful. Not everything is 100% accurate, but it gives a look at standing waves and why impedance matching is important.

[slug]

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,

Very good idea to have an emergency vhf ant

This ant would be best kept safely inside and unrigged

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

[tradrockrat]

Just to throw in my .02

I agree with a spare w/ cable tucked away nice and neat down below. I run a vesper AIS/VHF whip with the splitter (just installed - no review to give yet), but will definitely have a spare with a 20-ish foot cable that can plug into the splitter if the mast is gone or a handheld if lightening took out everything else.

The boat came with a Shakespeare and I was very happy to have it, until I found out it was dead. Why do I tell you this? Buy a new one or test that "lucky find" because a spare that doesn't work...that would be a very bad day.

[Richv]

You might also want to invest in a magnetic-mount 2m Ham Band antenna to keep in the closet. If your other antennae are disabled, just yank out the mag mount, snap it to a piece of metal, and plug it into your transceiver.

[NevilleCat]

something like this is neat and compact to store
https://www.whitworths.com.au/banten...enna-156-8-mhz

[tradrockrat]

Originally Posted by NevilleCat

something like this is neat and compact to store
https://www.whitworths.com.au/banten...enna-156-8-mhz

That. Is. Brilliant. Wonder how well it works.

[Richv]

Originally Posted by chrisr

hi all

here is a question for anyone who knows about vhf antenna

l<SNIP>
what would be the effect of having both antennas connected to the VHF at the same time ?

thanks in advance

cheers,

In broad technical terms, this is called a "phased array." You can't just take a Tee and join the two antennas and expect any kind of performance. First, almost all VHF transceivers and antennas have a characteristic impedance of 50 ohms. Tie two antennas in parallel, and you get 25 ohms and a bad mismatch. To connect two such loads, you need an impedance matching array. Now we need to allow for the way power adds. The length of the cables joining the antennas will determine the phasing of the signals, as will a receiver's distance from each antenna. In a few rare cases, this is usable; e.g., properly-matched and phased antennas can produce a directional pattern; a cardioid (heart-shaped) pattern is often used, as is a figure-8. In your application, you want the simple circular pattern of a single vertical antenna. In short, don't do it.

[continuouswave]

Originally Posted by chrisr

...a question for anyone who knows about vhf antennas...

I have some knowlege of antennas so I will try to answer your question.

Originally Posted by chrisr

...what would be the [effect] of having [two] antennas connected to [a transmitter] at the same time?

There is no single effect from what you propose. There are at least two effects to consider.

First, assuming the two antennas are connected simply in parallel, the impedance of the combination of the two antennas would no longer be the impedance of either antenna but some new impedance that would be different than either antenna. At the common point where the antennas are connected in parallel, the new impedance would then not match the transmission line impedance (which I assume was 50-Ohms).

This creates an entirely new effect, an impedance transformation due to the mismatch between the load impedance and the transmission line impedance.

The impedance presented to the transmitter will vary depending on the length of transmission line between the common point and the transmitter. The outcome will be that the transmitter will no longer see the 50-Ohm impedance it was designed to transmit power into. Since there are no output tuning controls in a VHF Marine Band transmitter, the effect will be the transmitter will probably produce less power output.

Second, having two antennas being fed by the same transmitter will create a new radiation pattern for the combination antenna, based on the the physical orientation of the antennas and the relative phase and power that is applied to each.

The more widely the two antennas are separated, the less effect they will have on each other. If the antennas are separated by many wavelengths, the effect of one antenna on the other will be decreased. (A wavelength at 156-MHz is roughly 1.9-meters; a rough formula is wavelength-in-meters = 300/frequency-in-MHz) If the antennas are close to each other, the effect of one on the other increases.

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.

It is extremely common that two (or more) antennas are connected to one transmitter to form what is known as a driven antenna array. In a driven antenna array there are precisely engineered elements of the driven array that take into consideration the common point impedance, the physical arrangement of the antennas, the power division among the antennas, and the phase relationship between the antennas. But none of this happens randomly and by chance, as would be the situation proposed for your boat, two antennas, and one transmitter.

There are many replies in this thread from people who probably have a great deal of long-distance cruising experience. I do not have much long-distance cruising experience, but I am quite familiar with antennas of the driven array type from 40-years as a broadcast engineer. I believe I have answered your question.

[toda]

I have two separate antennas connected to my VHF, one mast head, and one on the arch... I have a simple coax switch similar to this https://www.westmarine.com/buy/shake...lector--528786 I sometimes use the lower antenna to reduce my range when I am coastal cruising and don't want to be bothered by far off radio traffic. The switch is bulkhead mounted next to the radio making it easy to select antennas on the fly.

[Stmurray62]

Two antennae to one radio will work IF you utilize a “coax splitter” devise available at west marine.

Also, if you get de masted, you may have bigger issues than getting out a vhf radio message; might be a good idea to activate the EPIRB or personal locator???