Jump to content

Recommended Posts

 

what, just trailing it behind you ? 

 

rings of a multiturn loop in the empenage are probably the least trouble.

 

need to be cognescent of inducted currents, causing high voltage points, potentially arcs setting fire to the fibreglass etc

 

Needs quite a bit of care, and ingenuity to be effective in these years of low sunspot number and hence low maximum usable frequency.

 

With the low frequencies, the chances of getting any sort of decent current in a teleflex cable in a  jab is low/negligible. at least up to about 15 MHz.

 

Potential for the aileron control cables and arms is probably the most likely ,  still that wont happen until at least 14 MHz. 

 

Need to check for compatibility with any electronic assted ignition  system.

Link to post
Share on other sites

Trailing a wire was common practice in the 2nd world war. No different than a glider tow rope from the tug except you wind it in and out.

 

I used to supply radio systems to aircraft ferry pilots most had a relay style tuner with a wire they would run from inside the cockpit to the top of the fin then out to a wingtip. That would usually give you enough wire out to make reasonable contacts on HF

 

Most of the modern relay tuners only require around 10 to 13 mtrs of wire hung off them to operate ok

  • Agree 1
Link to post
Share on other sites

What about a helical similar to the HF helical ants I use on the amateur bands? Although I personally would be worried about RF getting into stuff it shouldn't. Burned out my home burglar alarm due to HF transmissions (100WPEP).

 

Link to post
Share on other sites

reasonable for a metallic body aircraft. the helical whip is a low impedance feed and certainly needs a good counterpoise. Now, what you CAN do is "wind up" that counterpoise with another helical whip as a loaded counterpoise.

 

In a composite aircraft, I think some really uber thin copper tape up the inside of the empenage to the front of the aircraft etc etc could perform the counterpoise job but it really needs to encase the aircraft  in rings and longitudally so that unfavourable outcomes due to high electric fields can not occur.. Antennas such as loops don't have any 'ends' and so a counterpoise is not required. Some shielding of the electricals may still be required for the composite aircraft.

 

 

Link to post
Share on other sites

With the cost and reliability of Sat Phones these days why would you even bother???? Have used HF A LOT over the years. Its nice and reliable and free to use but never seen a small lite set-up....

 

 

  • Like 1
Link to post
Share on other sites

end fed half wave is fine if it can feed "push" against something. like a metallic airframe, or simply lots of conductive control cables etc.

 

In simple terms

 

Antennas, if single ended , end fed must "push " against something to get the desired result. 

 

Centre fed antennas work by pulling and pushing in opposite directions simultaneously

 

Electrically small loops are a different and (ideally) assume uniform current  around the circumference - IE like an ADF loop antenna

 

-glen

 

 

 

Link to post
Share on other sites

ha ha

The Sav being a big metallic slab would function as a nice counterpoise for trailing an wire antenna out the back.

The large conductive surface area means good self capacitance 


as long as it doesn't get tangled in the elevator or rudder bits.....

 

Pilot and electronics will stay well shielded inside the aircraft, antenna trailing out the back should be very effective....

 

As for those loops Mark, I've always found it easier to get some 1.5, 2 or 3" heliax and use the nice full copper outer.  If you want to get clever , the inner can be used as one of the capacitors needed for resonance...

 

 

Link to post
Share on other sites

That loop does 3mhz to 11 mhz using a 15kv vacumm capacitor 500pf

Its run with a auto driver unit I made it has a stepper motor driving the tuning capacitor.

It is probably the best HF antenna I have used. Just incredible because of the super narrow bandwidth it just kills all the noise.

They are just unwieldy to try to fixture and rotate but they work best at only about 1 mtr from the ground. I made mine from 50mm alu tube as you can see. because they work on skin effect it really doesnt matter what metal you seem to use but as I found out the diameter of that material does matter. This loop should have worked to around 15mhz but after I had played with it and did some further experiments a much smaller diameter would have done the job. maybe a loop from 12mm tube not 50mm tube

220665403_IMG_34172.thumb.JPG.06453dc30ffbd7dbb6980487ad25d1e8.JPG106858564_IMG_37792.thumb.JPG.7e27882d3f1a4427644ec74d33c4d3dc.JPG

 

Link to post
Share on other sites

Nice implementation Mark.  I like the welded tabs for the vac.

 

Yeah, just keep the HV end (cap) at the top , away from the lossy ground.

Despite the relatively poor efficiency compared to a dipole in free space, the performance of loops is relatively good ( compared to the losses for a lowish dipole )  and as you point out, it works as a front end pre-selector for your receiver (and TX) and assists the radio performance (noise contribution outside intended passband) .

Remember efficiency is roughly circumference to the fourth power !

So, I think you are right to build it big. Once the circumference is > about 30% of a wavelength, the current will not be uniform over the circumference  and it will stop working as a mag loop and lose some if its directional properties.

While you might not notice the reduction in efficiency on receive, (since galactic and atmospheric  noise dominates), the TX efficiency will drop quite quickly as the loop is near the ground because the loop current strays from uniform, and also the dielectric losses of ground will rise,  and the unbalance current will rise (since power will be lost into the ground) ......all in all reduces the Q quite a lot. 

 

Of course if you build it "too good" there isnt enough useful bandwidth for many applications....

 

Have built loops to 75kW and 6m diameter, 3" copper at 1.5 MHz .  Efficiency is good > 1 diameter off the ground. 

-glen

 

 

 

 

 

Edited by RFguy
Link to post
Share on other sites

I'd say that aircraft with the gy-normous loop was functioning as a big metal detector.

 

Penetration of seawater  like that at LF roughly equal to loop size.... enough to spot an U boat cruising with snorkel

 

 

 

 

 

Link to post
Share on other sites
7 hours ago, Kyle Communications said:

That loop does 3mhz to 11 mhz using a 15kv vacumm capacitor 500pf

Its run with a auto driver unit I made it has a stepper motor driving the tuning capacitor.

It is probably the best HF antenna I have used. Just incredible because of the super narrow bandwidth it just kills all the noise.

They are just unwieldy to try to fixture and rotate but they work best at only about 1 mtr from the ground. I made mine from 50mm alu tube as you can see. because they work on skin effect it really doesnt matter what metal you seem to use but as I found out the diameter of that material does matter. This loop should have worked to around 15mhz but after I had played with it and did some further experiments a much smaller diameter would have done the job. maybe a loop from 12mm tube not 50mm tube

220665403_IMG_34172.thumb.JPG.06453dc30ffbd7dbb6980487ad25d1e8.JPG106858564_IMG_37792.thumb.JPG.7e27882d3f1a4427644ec74d33c4d3dc.JPG

 

As a radio tech and Ham I shouldn't be, but I am. Confused. You said it does 3mhz to 11Mhz then said it has a super narrow bandwidth. That would be a good antenna to use on a block where the neighbors bitch about towers cause you could keep it below the fence line. Another 'retirement project' I think...

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
×
×
  • Create New...