SSB Receiver - how to connect backstay antenna

[Babylon]

Basic electronics question: how do I connect the backstay as an antenna for my NASA HF3W SSB Receiver (ie no transmissions)? The unit came with a coax cable connected originally to an electronic antenna on the pushpit, which is now damaged and unusable, and I understand that a backstay works better anyway.

1. Do I clamp just the inner core of the coax cable to the backstay, just the outer shielding, or both?
2. Do I need to ground the receiver, and how does one do this on a boat? The engine earth?

At this stage, I just want to be able to receive AM/LW radio stations (eg R5 sports, R4, Froggie radio stations, etc), but later I want to experiment with connecting up to a laptop via the audio cable, so as to experiment with weather fax, NAVTEX etc.

Anything else I need to understand?

 

[tawhiri]

You can only use the backstay as an antenna, if it is isolated at the top and bottom, which is unlikely to be the case. The easiest solution is to pull a 10m length of wire up with the topping lift and connect the other end to the radio inner core. This antenna will be optiised for 7MHz. You do not need to worry about a ground unless transmitting.

 

[oldvarnish]

You can only use the backstay as an antenna, if it is isolated at the top and bottom, which is unlikely to be the case.

Strictly speaking, that's true. But the OP is only wanting to receive high powered broadcast stations, and in most situations a metal coat hanger would do that. I'd clamp the inner core to any bit of the rigging that's convenient and see what happens. Chances are it will do that job.

 

[Skylark]

If you only want broadcast reception, at least in the first instance, I'd simply clamp the coax core to the nearest, convenient part of the standing rigging. For now, ignore the shield. Simply test the reception to see if it works to your satisfaction.

The "fun" side to hf comes into its own when you need to transmit, without blowing the output stage of the transmitter due to mismatched load of the antenna. This is when you need to worry more about ground planes, ATUs and so on.

A bit of very basic theory is worth knowing. The product of frequency times wavelength is a constant, 3 x 10^8 (velocity of propogation).

So, 30MHz is 10m. 20MHz is 15m and 14MHz (amateur band) is known as 20m.

The fundamental antenna is a half wave dipole. In a boaty application, the equivalent dipole would be a quarter wave length of wire (standing rigging) and a ground plane or earth.

If you want to receive a 30MHz transmission, your wire should therefore be about 5m long.

Having said that, to pick up a high powered broadcast station, the metal coat hanger would probably work

 

[MM5AHO]

I agree with the "try anything" approach. For receive only, almost anything might work, so any connection into the mast (unless wood), the rigging etc will probably work. The more metal in the air connected to your coax centre core the better.

 

[srm]

One of my backstays is insulated so I just clamped the coax core to the stay between the insulators. Gives reasonable reception with HF3W and I can download Norwood weatherfax with my antique Win 95 laptop using an audio jack from the radio.

 

[Neil_Y]

I found wrapping an insulated wire up and around the backstay for ~3m, connected to the coax core where it came through the hull resulted in less interference from other electronics and lights on board than if I connected the wire to the rigging. That was with an HF3.

 

[Babylon]

Fascinating stuff. Thx for the replies.

 

[KellysEye]

>Strictly speaking, that's true. But the OP is only wanting to receive high powered broadcast stations, and in most situations a metal coat hanger would do that. I'd clamp the inner core to any bit of the rigging that's convenient and see what happens. Chances are it will do that job.

I agree. Since the OP has a receiver as said neither a ground or an insulated back stay are needed.

 

[rob2]

Jut thinking that one common failure mode with the masthead VHF antenna is water ingress to the feeder co-ax. Ideally the feeder would be confined to below decks, so maybe it could be connected to the chainplate? I realise that there are purely mechanical connections abetween the chainplate and backstay, but as it is under tension that may be enough to maintain the electrical continuity. If so, then presumably the whole rig becomes the HF antenna - not tuned, but possibly acceptable as a receiving antenna?

Rob.