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Goudurix · 11-10-2012, 01:16

Rolf,

as for choice of one of the 2 isolated backstays to use, your choice should be guided by the frequencies you want to favour:

- the longer one will favour working lower frequencies
- the shorter one will be a bit less efficient on the really low frequencies (2-4 Mhz) but will favour low take-of angle radiation for long range DX.

As a matter of fact, you can ± calculate the upper frequency limit at which low take-off angle is still possible for a given length of antenna:

(300/freq in Mhz)*0.625 will give you the Maximum Antenna Length in metres for alow take-off angle radiation for a given frequency

Example: 17m band is the highest DX frequency you want to use.

(300/18,130)*0,625 = 8.83 metres MAL (not taking into account small effect of velocity factor)
MAL = the total length from the top of the isolated stay until the antenna connector on the ATU (so including the strecth of GTO-wire from the lower end of the isolated stay untill the ATU)

Including velocity factor effect you end up with ± 9.2m real wire length.

But on eg 4.4 Mhz marine band this antenna wire is only 0.13 wavelength so expect losing efficiency there...but the SGC-230 will tune it.

Say you isolated stay is about 13 m (your typical 43 feet) and you have 1 meter exra of GTO lading to the ATU = 14m total wire length:
Freq = (300/14)*0,625 = 13.4 Mhz.
This means you will already loose low angle take-off radiation for the widely used 20m-HAM band but will be ideal for the 12 Mhz marine band. 0.625 wavelength is also the antenna length at which you get highest gain concentrated in the low take-off angle lobe.
If you reduce the total antenna length to 13m, the 20m HAM band is spot on the high gain upper limit.

A 13m antenna is still about 0.19 WL on 4.4 Mhz so not so far from a 1/4 wave.

So far this theory ...to lead to the choice of the stay to use in your case.

Jan

SoonerSailor · 11-10-2012, 07:54

Might be another reason there are two insulated back stays of slightly different length. Perhaps a previous owner found his original insulated backstay was abt 1/2 wavelength on a band he
liked to use. This would have been difficult for most tuners to match. He then may have installed the second one and made it just different enough in length for the tuner to come up with a match.

Chip

tolly · 11-10-2012, 08:18

I will try to measure (or calculate) exactly how long the two "antennas" are on the boat and come back with that information.

Rolf

Goudurix · 11-10-2012, 08:36

....but you have to add the stretch of wire (typically GTO-15 high-voltage insulated wire) from the isolated stay until the antenna lug on the ATU which should be routed via standoffs etc to keep it away from the lower part of the stay, metal tubes especially grounded ones...

Jan

tolly · 12-10-2012, 07:15

Quote:

Originally Posted by Goudurix

....but you have to add the stretch of wire (typically GTO-15 high-voltage insulated wire) from the isolated stay until the antenna lug on the ATU which should be routed via standoffs etc to keep it away from the lower part of the stay, metal tubes especially grounded ones...

Jan

But if I add that cable length, there will be no difference if one backstay is shorter isolated than the other, since both will need to go down to the atu which is in the hull, and will then finally will have the same length.

I measured and calculated the backstay lengths and the starboard one originally connected to a SSB radio is about 1403cm (553") and the other one not connected to anything is then about 1323cm (521").

What can come out of this knowledge I don't know...yet...but I am looking forward to be enlightened.

Rolf

drruss · 12-10-2012, 07:26

I have a Yaesu 857 with the FC
-30 and love it. No problems at all. It requires very little space and the face is removable for remote installation. I have been very pleased with this unit. KF5IXL

btrayfors · 12-10-2012, 07:27

Use one. The one nearest the tuner (wherever you're going to locate that under deck).

No need for two and no likely advantage.

NB: the physical length of the antenna, including the GTO-15 feedline, is not the same as the electrical length.

To calculate the electrical length you'd need to know the velocity factor of the backstay and the GTO-15 feedline. That's not necessary and won't help you anyway....the backstays and insulators are already in place.

Hook 'em up and see what happens.

Bill

Goudurix · 12-10-2012, 07:55

Indeed like bill says ....but you will at least get an idea using the formulas.

Let's assume you use the 14.03m backstay, and you have 1m of GTO-15 connecting to your atu:

15.03 metres
Velocity factor ± 0.96 (oK....I know...just a quick assumption...)
=> 15.03/0.96 or 15.65 m electrical length

300/15.65*2 gives you: kit is a halfwave on 9.58 Mhz => you might get troubles to tune on 30m HAM band

300/15.65*0.625 gives you: 11.98 Mhz is ± your upper limit for low take-off angle radiation.....and the 12.3 Mhz marine frequency is just above that.

For frequencies above 12 Mhz you will increasingly get "cloud warming lobes" hence suitable for NVIS or closer range DX.
But this is theory and in practice, you might still get long range DX. But if you really mean to work mainly Ham 20m and 17m DX and up, you should ahev a shorter antenna system to be used with the ATU.
Or dedicated vertical dipoles nnext to the backstay antenna system, eh Bill?

As an indication.....

Jan

btrayfors · 12-10-2012, 08:04

Yes, Jan, exactly.

If you want to work DX, definitely also consider single-band vertical dipole(s) on 20m and higher. I rig them forward...between the forestay and the mast. Details on my website: Marine Antennas

But, for your main all-band antenna, the backstay is definitely the way to go.

Bill

Goudurix · 12-10-2012, 08:07

Bill I was able to sell the Yaesu FC-40 atu....

btrayfors · 12-10-2012, 08:08

Good!

SoonerSailor · 12-10-2012, 08:24

Quote:

Originally Posted by drruss

I have a Yaesu 857 with the FC
-30 and love it. No problems at all. It requires very little space and the face is removable for remote installation. I have been very pleased with this unit. KF5IXL

The FC-30 has a very limited matching range. Good for matching a near resonant antenna, but not so good for a backstay that is to be used on many bands.

Of course, if your coax is very lossy, even a limited range tuner has a better chance of coming up with a "good" SWR.

tolly · 12-10-2012, 16:03

Quote:

Originally Posted by Goudurix

Let's assume you use the 14.03m backstay, and you have 1m of GTO-15 connecting to your atu:

15.03 metres
Velocity factor ± 0.96 (oK....I know...just a quick assumption...)
=> 15.03/0.96 or 15.65 m electrical length

Jan

Two things:
1. According to where I can mount the atu, the length of the connection cable would be about 2m. I thought it wouldn't matter particularly, with a working atu.
2. What is the difference between a GTO-15 cable and an ordinary spark plug cable for a car or motorbike, that is supposed to isolate at least 30 kV? Maybe the UV resistance but I'm not sure of that either, since used on a motorbike it definitely is exposed to sunshine.

Rolf

Goudurix · 15-10-2012, 01:06

Hi Tolly,

1. GTO-15 cable: I guess some other guys will be able to answer if a motorbike spark plug cable has the same properties. I guess so. I use the inner lead of RG-213 coax as the connection between the backstay and the ATU. It has a heavy section of copper lead and the insulation should be adequate. (of course the outer insulation and brais are removed. Disadvantage is it is not UV-stable, tends to crack so I covered it with balck tape

2. Maximal frequency and Halfwave frequencies:

a) your 14.03 m backstay + 2.00m GTO-15 = 16.03m (16.7m electrical length)
Your theorethical upper frequency limit (for low take-off angle radiation) is 11,2 Mhz.... so it really favours lower marine SSB frequencies until 8 Mhz and 80,40 and 30m Ham.
It is a halfwave for: 8.9 Mhz so quite close to 8 Mhz marine band, but I guess the SGC-230 will be able to tune it

b) your 13.23m backstay + 2.00m GTO-15 = 15.23m (15.9m electrical length)
Low take-off angle MUF is 11.8 Mhz. Given the slope, I think 12 Mhz marine band might still give acceptable long range DX. 20m Ham band? To be tested.
It is a halfwave for 9.4 Mhz so no issue for 8 Mhz marine band.
I would suggest to use this one, and see what it gives on 12 Mhz, 14Mhz Ham and above.
If necessary you could easily build quick-deployable vertical dipoles (see Bill's links) for 12 Mhz and 14 Mhz, for which you do not need the ATU. The combination of these would give you good allband use until 11 or 12 Mhz on the backstay sloper and very good long range DX with the vertical dipoles. above 11 Mhz.

Jan