VHF and UHF

[effinqueue]

I have a Pelican PL which has a composite body with aluminium wings.

I'm halfway through repairing damage to the bottom of the fuselage. I won't go into how it was damaged at this point but as well as damaged fibreglass it has cut both the coax cables to each antenna.

I have both the normal VHF comms radio and also a UHF unit fitted. Both were fitted by the previous owner.

There is a dipole type and also a stub type at the top with it's ground plane attached underneath inside the plane.

I don't know which radio was connected to which antenna. Does it matter? 

Should I just ditch the UHF?  

 

[RFguy]

Given the simple construction of the antennas, the UHF antenna will be the  smaller.  got any photos ?

[effinqueue]

 

underneath top stub antenna

 

 

Dipole with copper legs 

[skippydiesel]

The UHF antenna probably doesn't have much of a ground plain , if any. 

 

In a composite or wood aircraft, the VHF (not the dipole) antenna should have a significant ground plain - fit one it will leave the dipole in the shade for both transmit & receive..

[cherk]

6 hours ago, effinqueue said:

 

underneath top stub antenna

 this is the UHF one

 

Dipole with copper legs 

this one is the VHF

[RFguy]

hmm OK the dipole is definetly VHF

 

the stub end fed antenna with the ground plane- that ground plane looks like it could be used for VHf or UHF, so it depends what is on the other side of the fibreglass. if it is ~ 16cm long skinny , that would be a UHF stubby whip.  If it is perhaps 30cm long and appears to have a coil or spring in it, probably VHF. 

 

-glen

RF Engineer.

 

[old man emu]

Question from one who knows nothing about radio transmission.

 

Both sets of wires seem to be attached simply to metal straps, which I assume form the antenna ground planes. The strips seem to be glued to the body. If a large area sheet of metal replaced the thin strips, would that improve the trans-ception capabilities of an aerial? On the converse, would a large ground plane require some mans of screening it from the aircraft's electrical signal (engine ignition system, electronics etc)

[skippydiesel]

58 minutes ago, old man emu said:

Question from one who knows nothing about radio transmission.

 

Both sets of wires seem to be attached simply to metal straps, which I assume form the antenna ground planes. The strips seem to be glued to the body. If a large area sheet of metal replaced the thin strips, would that improve the trans-ception capabilities of an aerial? On the converse, would a large ground plane require some mans of screening it from the aircraft's electrical signal (engine ignition system, electronics etc)

Glen will have the technical answer  - mine is: ground planes can be made of any good conductive material (metal). Metal aircraft (& carbon fiber) make excellent ground planes. Composites/wood must have constructed/fabricated ground planes, these are usually made of thin metal. The ones shown are  copper ribbon . I have used builders aluminium flashing to good effect.  A flat symmetrical plate (or cross) with the antenna in the centre is the best.  Flexible material can be molded & glued to the inside of the fuselage. I have forgotten the formula for the optimum size, this is often compromised by the available space

[RFguy]

for small ground planes, like these, being a solid or approximated with thin elements wont make any difference in this application.

the only time this is important is if, for example, you have some sensitive electronics underneath the antenna- the solid plane will be an electrostatic shield and prevent possible interactions.

also, noise from the cockpit can get on the coax cable outer and be brought into the antenna pickup field. The complete ground plane (solid) reduces the pickup from garbage on the cable. 

 

in this application, the minimum you need in thin elements is two wires in opposite directions to acheive symmetry.

 

The coax cable should come away at 90 deg to the antenna, ideally, in the two wire scenario.  

 

Edited December 2, 2022 by RFguy

[effinqueue]

Hey thanks for confirming this. I have a small approx 150mm long skinny stub aerial on top so as Glen mentioned most likely UHF.

I'll be reconnecting the broken coax with BNC connectors. That should work OK do you think?

I'll find someone to check the SWR before use. Anything else I need to consider?

 

 

[RFguy]

yeah , UHF.  BNC is fine . make sure you buy 50 ohm connectors, not 75 ohm BNCs....

 Be aware, when you talk on UHF from 1000' above the ground, you'll be heard 150km + ....
you might find it easier to place the whole run, instead of a join...

 

https://www.minikits.com.au/BNP01
and
https://www.minikits.com.au/BNS02

https://www.minikits.com.au/HT336
and

https://www.minikits.com.au/JAW336D2

 

https://www.ebay.com.au/itm/321637603591

Edited December 3, 2022 by RFguy

[skippydiesel]

Just wondering why you would want a built in UHF -  Understandable, if you are in regular communication with non aviation  persons on the ground (property owners, etc). I have found a hand held UHF and a mobile phone are all that I seem to need, on those infrequent times, when trying to make contact with those below.

[old man emu]

5 hours ago, skippydiesel said:

I have found a hand held UHF and a mobile phone are all that I seem to need, on those infrequent times, when trying to make contact with those below.

I have found a hand held Bible and a string of beads are all that I seem to need, on those infrequent times, when trying to make contact with those above.

[pmccarthy]

Telephone to glory…

[old man emu]

10 hours ago, pmccarthy said:

Telephone to glory…

Would Jehovah's Witnesses reject a party line?

[RFguy]

UHF is useful. SKippy- most planes I know (jabirus aside) there is no way to listen to a portable inside the running aircraft....  and its highly unlikely anyone is going to hear you over the noise. Hence plumbed-in UHF radio is a good idea ...... (my new radio has UHF built in- delayed due to parts unavailability) - My Jabiru 230 though, you could use a radio without a headset. Very quiet. Compared to a rattling and dinning (but very mice to fly)  RV6A, it's peaceful.  I'm guessing the heavy fibreglass  around the pax cell doesnt vibrate much  in the acoustical spectrum.  . 

[skippydiesel]

Must admit my Zephy was very quiet inside (compared with GA types) and being composite, transparent to radio transmissions. I found my phone could be tucked between ear & headset and function quite well. UHF has a mike extension so again speaker held to ear while mic in front of mouth. New aircraft, metal, may not be so accommodating.

[facthunter]

When the Cessna's first came along they didn't use headsets. Just the  speaker, The earlier 172 had the six cyl Continental  0- 300. The 175 had a geared version.  Nev

[skippydiesel]

12 minutes ago, facthunter said:

When the Cessna's first came along they didn't use headsets. Just the  speaker, The earlier 172 had the six cyl Continental  0- 300. The 175 had a geared version.  Nev

We was tough (and going deaf) back then

[facthunter]

Tha cabins were quite well sound proofed. The bigger Auster with four stub exhausts was bad for the ears with a fabric covered cockpit.  Nev

[pmccarthy]

Never used headsets with Cherokees in the bad old days.

[jackc]

I was thinking of putting my Ham radio in my Super Bin Chicken, with small horn speaker above my left ear and use a noise cancelling mic where its easy to get at.   Want it standalone and not integrated to the Avionics at all.