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Old 05-09-2018, 19:45   #1
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HF and VHF antenna proximity

Hi all,

Can anyone tell me if there is a rule of thumb about the proximity of a VHF antenna and a HF antenna?

Iíve got a spot I really like for the spare VHF whip, but it would be reasonably close to the HF backstay antenna. (About 1 meter apart). HF is a 150 watt transmitter, VHF is 25 Watts, whip is 1.8 meters.

Matt
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Old 06-09-2018, 00:47   #2
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Re: HF and VHF antenna proximity

That's about the spacing between my cockpit VHF and backstay HF antenna and it appears not to have caused any problems. As a single hander I never use them at the same time though although I have used the HF with the VHF switched on on standby. I tend to the opinion that neither transmits with enough power to induce much of anything in the other.
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Old 06-09-2018, 00:56   #3
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Re: HF and VHF antenna proximity

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Originally Posted by GILow View Post
Hi all,

Can anyone tell me if there is a rule of thumb about the proximity of a VHF antenna and a HF antenna?

I’ve got a spot I really like for the spare VHF whip, but it would be reasonably close to the HF backstay antenna. (About 1 meter apart). HF is a 150 watt transmitter, VHF is 25 Watts, whip is 1.8 meters.

Matt
The rule of thumb is "as far apart as practicable".

The VHF antenna won't affect your HF (Tx or Rx) in any meaningful way and the only time the HF might degrade (but probably won't degrade) your VHF is when the HF is transmitting.

However it is unlikely you will be needing the VHF while transmitting on the HF and as you state, the VHF is a spare and thus presumably unconnected most of the time.

Additionally, VHF is vertically polarised and I presume your spare VHF will be vertical while your HF backstay is inclined - again this increases radio separation.

FWIW, I have fitted them much closer on aircraft without issues.
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Old 06-09-2018, 02:17   #4
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Re: HF and VHF antenna proximity

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As a single hander I never use them at the same time though although I have used the HF with the VHF switched on on standby.
Ummm... I feel silly now. Hadn't thought about the single handed aspect. Good point.
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Old 06-09-2018, 02:18   #5
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Re: HF and VHF antenna proximity

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The rule of thumb is "as far apart as practicable".

The VHF antenna won't affect your HF (Tx or Rx) in any meaningful way and the only time the HF might degrade (but probably won't degrade) your VHF is when the HF is transmitting.

However it is unlikely you will be needing the VHF while transmitting on the HF and as you state, the VHF is a spare and thus presumably unconnected most of the time.

Additionally, VHF is vertically polarised and I presume your spare VHF will be vertical while your HF backstay is inclined - again this increases radio separation.

FWIW, I have fitted them much closer on aircraft without issues.
Thanks Wottie, it sounds like I don't need to worry about that excellent Icom and ATU on the boat nuking the VHF then.
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Old 06-09-2018, 02:23   #6
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Re: HF and VHF antenna proximity

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However it is unlikely you will be needing the VHF while transmitting on the HF and as you state, the VHF is a spare and thus presumably unconnected most of the time.
I probably shouldn't say spare, I should have said secondary. I want to use the second VHF when I am at anchor, as climbing up to the cockpit any time something interesting happens on the radio is hard work, and quite frankly, a bit dangerous.

So the second antenna will most likely remain attached all the time, but the radio to which it is connected should be switched off and only be used at anchor. (The cockpit VHF has the masthead antenna connected to it.)

But from what you are saying, the proximity and angles make it unlikely the HF will fry the VHF from that range. I'll go back to my books and remind myself about the meaning behind polarisation in the antenna context, I have forgotten.
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Old 06-09-2018, 02:44   #7
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Re: HF and VHF antenna proximity

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I probably shouldn't say spare, I should have said secondary. I want to use the second VHF when I am at anchor, as climbing up to the cockpit any time something interesting happens on the radio is hard work, and quite frankly, a bit dangerous.

So the second antenna will most likely remain attached all the time, but the radio to which it is connected should be switched off and only be used at anchor. (The cockpit VHF has the masthead antenna connected to it.)

But from what you are saying, the proximity and angles make it unlikely the HF will fry the VHF from that range. I'll go back to my books and remind myself about the meaning behind polarisation in the antenna context, I have forgotten.
So you don't have a remote mic for your primary VHF then .

Certainly won't be an issue when the secondary VHF is turned off!
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Old 06-09-2018, 02:46   #8
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Re: HF and VHF antenna proximity

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So you don't have a remote mic for your primary VHF then .
I see what you did there.
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Old 06-09-2018, 02:48   #9
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Re: HF and VHF antenna proximity

Nothing wrong with having a second VHF, I'm all for redundancy if you have the room
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Old 06-09-2018, 02:57   #10
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Re: HF and VHF antenna proximity

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Nothing wrong with having a second VHF, I'm all for redundancy if you have the room
Particularly when you need to be a mountain goat to get from the cockpit to the cabin.
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Old 06-09-2018, 10:42   #11
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Re: HF and VHF antenna proximity

Hi All,


The technical answer to antenna spacing or spacing to a metal object is: Minimum spacing should be 1/4 wavelength at the lowest frequency. For HF it would be half way up the boat. Too far to be usable. For the VHF it would be about 20 inches.


So, since we can't accommodate the HF separation, we can at least get the minimum 20 inches for the VHF.


The quarter wavelength allows an impedance transformation from low to high which makes the antenna in questions see the nearby metal as a high impedance (high resistance which is low load). The tuning of the antenna may be affected, but it usually won't be drastic.


Near field is considered to be 2 wavelengths. Beyond that distance, tuning usually won't be affected.


Wavelength is calculated as the speed of light divided by the frequency.


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Old 06-09-2018, 11:24   #12
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Re: HF and VHF antenna proximity

GI-
One additional consideration, especially for the "spare" radio, but really for all radios that are not in use. The coax should be unplugged from the rear of the radio, or where it enters the deck, and plugged into a lightning round when the radio is not in use.
This ensures that it will "normally" go to ground, and not to the radio. So if you're not on the boat, or you're busy with other things, and lightning DOES strike? It can't comes down that cable and take out your radios.
Most sailors don't bother, but for many radio ops on land, this is a normal precaution.
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Old 06-09-2018, 16:27   #13
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Re: HF and VHF antenna proximity

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GI-

One additional consideration, especially for the "spare" radio, but really for all radios that are not in use. The coax should be unplugged from the rear of the radio, or where it enters the deck, and plugged into a lightning round when the radio is not in use.

This ensures that it will "normally" go to ground, and not to the radio. So if you're not on the boat, or you're busy with other things, and lightning DOES strike? It can't comes down that cable and take out your radios.

Most sailors don't bother, but for many radio ops on land, this is a normal precaution.


A good suggestion. Manera was hit by lightening once before, right in front of the POís eyes. (He was driving the Russel Island ferry past her mooring). The vhf masthead antenna ended up ďlike a little squiggly pig tailĒ in the tender on the davits and the vhf radio was as dead as a dodo.
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Old 06-09-2018, 16:30   #14
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Re: HF and VHF antenna proximity

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Hi All,


The technical answer to antenna spacing or spacing to a metal object is: Minimum spacing should be 1/4 wavelength at the lowest frequency. For HF it would be half way up the boat. Too far to be usable. For the VHF it would be about 20 inches.


So, since we can't accommodate the HF separation, we can at least get the minimum 20 inches for the VHF.


The quarter wavelength allows an impedance transformation from low to high which makes the antenna in questions see the nearby metal as a high impedance (high resistance which is low load). The tuning of the antenna may be affected, but it usually won't be drastic.


Near field is considered to be 2 wavelengths. Beyond that distance, tuning usually won't be affected.


Wavelength is calculated as the speed of light divided by the frequency.


Tim Chapman
Owner
arrowantennas.com


Thank you Tim, clearly this is your field of expertise.

Can I please ask for a clarification? From what you have written it seems the focus is the implications for tuning? Can you see the potential for damage to either radio from the transmissions of the other radio and if so, what would you do to mitigate that risk?
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Old 06-09-2018, 18:14   #15
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Re: HF and VHF antenna proximity

Hi Matt,

Real life experience: when we bought I-2, her VHF antenna was mounted on the outboard end of the solar/radar arch, about 1.3 m from the backstay. At times I use the backstay for my HF antenna (mostly I use the entire rig, mast and all other wires except the backstay). I run from ~ 30 to ~80 watts, mostly on 40 m and sometimes 20 or 80. There has never been any problem with the connected VHF rig.

These days my primary VHF antenna is at the masthead, but I have used the lower one for AIS, and there has been no problem with that unit either.

As always, FWIW!

Jim
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