Whip antenae

GaryC · 08 October 2009, 12:21

Hello

My name is Gary and I am new to this forum. So if doing this wrong, apologies.
I am looking at changing my ariel to a bigger 2.7 metre whip. Glomex appear to do a nice one but it appears to come with only 4.5metres of cable.

Does anyone know where I can get a 2.7 metre whip with 10 or more metres of cable. (4.5 just is not enough)

One answer is probably to put a cable connector and add some cable. This has two problems. first is that if you add a connector then you lose 3db in signal strengh. Which pretty well negates the use of the large whip. Probably more importantly the joint would be on the floor and would possibly allow water to enter. (even with amal tape wrapping.)

Any ideas?

Thanks

Gary

JSP · 08 October 2009, 12:59

Gary, Welcome to RIBnet. Contact MikeCC on here http://rib.net/forum/member.php?u=1880

jyasaki · 08 October 2009, 16:27

When I bought my Shakespeare, I think it came with a 20' cable attached (along with the solderless PL-259 connector, which, to my surprise, has been working quite well for a couple of years.) Not quite the length you're looking for, but better than 12'...

Easy enough to join a cable by installing either a PL/SO-259 connector pair, or go BNC with a barrel connector. If the splice will be out of sight, wrap the entire connection with self-amalgamating silicone tape to seal it up.

jky

donegaldan · 05 November 2009, 10:54

Although the bigger antenna will work slightly better, the rule of thumb
with vhf (line of sight) is that higher is better. I would consider a higher
mounting or some sort of extension to an A frame. Remember that steel will be less whippy than a fiberglass shroud.
Use the formula below to assist in the calc's but the size of the antenna is a minor point
went talking about signal propagation.

distance in miles = square root of (1.5 x height of the antenna in feet)
distance in kilometres = square root of (1.5 x height of the antenna in metres)

That is of course if the desired effect is better vhf com's as apposed to esthetics

Channel Ribs · 05 November 2009, 11:19

Quote:

Originally Posted by donegaldan

distance in miles = square root of (1.5 x height of the antenna in feet)
distance in kilometres = square root of (1.5 x height of the antenna in metres)

But the relationship between feet and miles is no the same as that of meters and kilometers.

If my antenna is a big 7ft job then (sq rt of 7x1.5) the range is 3.2 miles.

If my antenna is the same size measured in meters then (sq rt of 2.13x1.5) the range is 1.8km which is 1.1 miles.

The metric one should be 12.75 and not 1.5, I think.

I thought the best way of working out the value of a longer antenna was to take the height above sea level as two thirds up the length of the whip, therefore a 7ft one is about 4ft higher than a 1ft mounted in the same location.

In any event the effective height of the antenna is calculated by multiplying the square root of the height in feet by 1.23. So our fixed radio at work which is 50ft up has a radio horizon of 8.7 miles, assuming the receiving station is at sea level.

9D280 · 05 November 2009, 11:33

Gary,
Welcome to Ribnet.

And that's before you take the theoretical shape of transmission signal from a long vs a short antenna, which essentially means that if your boat is rolling you don't want a perfect "disc" of transmission as quarter of your signal will go skywards the other quarter downwards, so only half of tyour perimiter will get a decent signal. (same reason handhelds have tiny wee ones, 'coz you'll never hold it perfectly vertical)

Also at 2.5m, if you stuff the boat, you could get the aeriel in the back of the head if it's flexible enough!
(and the longer it is the quicker it;s likely to fatigue......)

donegaldan · 18 November 2009, 09:45

Quote:

Originally Posted by malthouse

But the relationship between feet and miles is no the same as that of meters and kilometers.

If my antenna is a big 7ft job then (sq rt of 7x1.5) the range is 3.2 miles.

If my antenna is the same size measured in meters then (sq rt of 2.13x1.5) the range is 1.8km which is 1.1 miles.

The metric one should be 12.75 and not 1.5, I think.

I thought the best way of working out the value of a longer antenna was to take the height above sea level as two thirds up the length of the whip, therefore a 7ft one is about 4ft higher than a 1ft mounted in the same location.

In any event the effective height of the antenna is calculated by multiplying the square root of the height in feet by 1.23. So our fixed radio at work which is 50ft up has a radio horizon of 8.7 miles, assuming the receiving station is at sea level.

My apologuies for not making my post clearer,
When i refer to "height of antenna", I'm talking about its height above water - not the antenna lenght.
To be perfectly honest, its not my calculations either, if you search for VHF propagation, you will find these, as they are taught internationally.
So sorry about that, but guess your big 7 ft Job will need to be recalculated to see how good it is.
This is also why coast radio stations are built on top of hills and up whopping great towers, to increase height, and to give better coverage to the horizon.
If you want to calculate 2 stations of differing height, i suggest you google "dipping height" and it will broadly allow you to do it, really for visual light but
between you and me you can add on a few extra meters for vhf and you'll be fine !

But it matters not how big your stick is, its how high you hold it up.
Good luck with the calc's again

GaryC · 18 November 2009, 12:27

Quote:

Originally Posted by jyasaki

When I bought my Shakespeare, I think it came with a 20' cable attached (along with the solderless PL-259 connector, which, to my surprise, has been working quite well for a couple of years.) Not quite the length you're looking for, but better than 12'...

Easy enough to join a cable by installing either a PL/SO-259 connector pair, or go BNC with a barrel connector. If the splice will be out of sight, wrap the entire connection with self-amalgamating silicone tape to seal it up.

jky

Thanks. I did consider this but was always told that introduction of a connector, drops the transmitted signal by 3db. (half). The 2.7 whip has in improvedment of 3db, so using a connector is basically negating the improvement.

Replacing the full cable would be a better option but I don't know whether this was an option on the glomex whip.

Gary

GaryC · 18 November 2009, 12:46

My original reason for getting a 2.7 metre whip was one of improved signal transmission. The longer whip is higher and has a flatter transmission footprint. And I already bought the bracket.

I was made aware by the launch site, that when 14 miles out and on an emerg call to the coastguard (which was fine reception wise) that the launch site could only hear one side of the conversation. I am hoping the bigger whip will give us a little more range.

I have found that Banten do one.

Many Thanks

Gary

9D280 · 18 November 2009, 14:11

Quote:

Originally Posted by GaryC

has a flatter transmission footprint. And I already bought the bracket.

Which may be to your tramsmitting detriment unless your RIB is perfectly stable & aligned to the horizon....... As you roll over waves that perfect disc will tip sky / sea ward along with the aerial, meaning the signal will fire over the top & miss most recieving antennae on one side & get swallowed by the sea on the other.

Brackets are remarkably adaptable.....
http://rib.net/forum/attachment.php?...4&d=1207258553

willk · 18 November 2009, 14:28

Quote:

Originally Posted by GaryC

I was made aware by the launch site, that when 14 miles out and on an emerg call to the coastguard (which was fine reception wise) that the launch site could only hear one side of the conversation. I am hoping the bigger whip will give us a little more range.

What channel were you working on, 16 or a Duplex* CG working channel, e.g. 23?

______________________
* Hint

GaryC · 19 November 2009, 15:30

Quote:

Originally Posted by willk

What channel were you working on, 16 or a Duplex* CG working channel, e.g. 23?

______________________
* Hint

Initially Ch 16 and then another channel that I cant quite remember. (it will have been one selected by the coastguard)

The main reason is probably one of ariel height. The Launch site are about 50ft above sea level. Whereas the Coastguard are on top of a cliff and then have a tower.

GaryC · 19 November 2009, 15:34

Quote:

Originally Posted by 9D280

Which may be to your tramsmitting detriment unless your RIB is perfectly stable & aligned to the horizon....... As you roll over waves that perfect disc will tip sky / sea ward along with the aerial, meaning the signal will fire over the top & miss most recieving antennae on one side & get swallowed by the sea on the other.

It is my understanding that the vertical divergence is approx 50deg for a 6db gain antennea. So unlikely to cause problem. (to much)

It will also be mounted on top of an A Frame so should be ok re height.

Thanks

Gary

willk · 19 November 2009, 15:43

Quote:

Originally Posted by GaryC

Initially Ch 16 and then another channel that I cant quite remember. (it will have been one selected by the

You will be aware that the CG often use Duplex working channels. Your Launch site would not be technically able to receive you on this channel, only the CG's replies. If they could hear you on 16 but not the other channel - this is probably the reason. They might have just missed the short initial transmission on 16 too.

Otherwise, it's your wee antenna