Best SWR achieved?

[AJ85WA]

Wirejock 1.05 to 1.5 is my SWR range

The ground plane is made up of 4 steel wires (550mm long) connected to a piece of aluminium the antenna is mounted to at the base.

[wirejock]

Ahhh. That makes sense. Duh. Sorry. That range is very good.

[Bill Boyd]

It's because you have drooped your radials away from the radiator, getting closer to 50 ohms impedance for your quarter-wave monopole antenna. As you dress the radials further away fom the radiating element so the they look more and more like the other half of a dipole (180 degrees between the two poles) your feed point impedance will go from 36 to 72 ohms. At a VSWR of 1.0:1, you've found the droop angle where it's 50 ohms. Not so easy to do using the belly of your monocoque fuselage as your ground plane, since the "droop angle" is fixed by Van himself

If you find a spot on the belly where the installation gives 50 ohms and 1:1, it was luck, and time to grab a beer.

-Stormy

[dpansier]

I’m surprised the vendor suggested cutting the element to change the VSWR without understanding the antenna construction. The antenna also appears to be stripped of the White Powder Coating on the element, did you purchase the antenna this way?
I would appreciate an email identifying the vendor so I can communicate with him.

Based on this mis-information, I will attempt to clarify the types of VHF aircraft antennas and their construction.

Three types of VHF aircraft antennas are used on general aviation aircraft, the Rod Antenna, Pi-Network Antenna, and the Shorted Stub Antenna.

The simple rod antenna was commonly used in the early days of VHF aircraft communication, the antenna consisted of a bent metal rod with a feed thru insulator allowing the antenna to be mounted on the aircraft belly. The co-ax was stripped and the center conductor was attached to the mounting nut. The co-ax braid was grounded to the aircraft fuselage or ground plane.
With most of the activity in the 118 to 127 MHZ range the antennas frequency response was more than adequate to cover the frequency and still have tolerable VSWR.

As the air traffic increased and more ATC facilities came on line, the frequency range was increased to 118 to 137 MHZ. The simple rod antenna proved unable to cover the wider range with reasonable VSWR performance.

Antenna designers went to work and developed matching networks to increase the antennas frequency response and meet the needs of the new wide band transceivers.

The Pi Network and Shorted Stub designs are widely in use today. Examples of the three types are listed below.

Simple rod antenna

http://www.aircraftspruce.com/pages/..._com/av534.php

Pi Network Antennas
http://www.aircraftspruce.com/pages/..._com/av-17.php
http://www.aircraftspruce.com/pages/...m/av10_ant.php
http://www.aircraftspruce.com/catalo.../comant121.php
http://www.aircraftspruce.com/catalo.../comant122.php

Shorted Stub Antenna
http://www.aircraftspruce.com/pages/...om/dmC63_1.php
http://www.aircraftspruce.com/pages/..._com/c63_2.php
http://deltapopaviation.com/VHF_Com_Antennas.html




On the Rod antenna, a shorter element length will increase the resonant frequency, adding element length will decrease resonant frequency.
It is not that simple on the antennas with a built in matching network.
Both the Pi Network and the Shorted Stub antenna performance depend on the inductor / shorted stub relationship to the element length, changing one with out changing the other will degrade the wide frequency response of the antenna.
If the intent is to limit the frequency to a very narrow range, cutting the element length may work out but don’t expect good performance at the band edges.

Checking the antenna installation if fine with a VSWR device to insure it is in the spec the manufacture listed. Both Dorne & Margolin and Comant lists VSWR’s of < 3.0 for the bent element and < 2.0 for the straight element antenna.
Changes to the antenna design and network relationship and expecting wide frequency response out to band edges will require much more sophisticated test equipment.

Keep in mind when testing antennas that the Pi Network designs will show open when inserting a Volt Ohm Meter (VOM) across the BNC connector and the Shorted Stub will show a short.

By the way I’m not aware of any bent element VHF Com antennas designed for general aviation VHF Com that offer VSWR’s less than 1.4 over the entire 118 to 137 MHZ range. If anyone is aware of one, I would like to learn about.

[AJ85WA]

Hi Guys

Just to clarify
My supplier never asked me to tune my antenna. I also never said they did?

I am getting desperate to get my comm working as flying is not enjoyable the way it is now, so with my lack of knowledge I started experimenting with the antenna because I was advised by the supplier that 1.8SWR is not acceptable and causing my radio issues

They also advised the following
It is unusual to mount the VHF antenna at the bottom of the aircraft (not illegal, just unusual)

SWR of 1.8 is not very good. It's just good enough to not blow up the transmitter.

In my experience practically all TX issues I have so far worked on myself are due to incorrect ground planes


So the way I see it I have checked:

Ground plane ok - metal aircraft with mounting screws making good connection

Coax tested with dummy load and SWR showing 1.03

Antenna tested giving 1.8 SWR at worst

Could it actually be the radio?

[krw5927]

Originally Posted by AJ85WA

Could it actually be the radio?

Short answer: YES!

[airguy]

I achieved a perfect SWR of 1.00 a while back...

... and then realized my meter was dead....

[flightlogic]

You hit the jack pot here on the thread. Posters have taken more time than I ever saw....even on a ham radio forum, to illustrate the design and differences of VHF whips for GA. Buy them a beer!
Back to business.... I think you can take your test jig apart. Your plane doesn't have drooping radials. It also will have an element pretty close to earth when you call ground control. Lots of variables will be different. Simply bond the antenna you choose (and save money over Spruce) and buy Delta Pop. Ground bonding, coax fittings and quality wire make the most difference of all.
A modern radio will protect itself from reflected power (which manifests as heat in the final amp transistors). The blowing up phrase is quite misleading.
And while you have been lead down that path, you might have missed root casuses of noise and echos etc. You need to begin at the radio, MIC levels, Side tone levels, A bench test if needed to see if the radio is in alignment.
Even operators have been caught up not knowing that a typical mic, such as a Lightspeed Zulu have a tiny screw and potentiometer on it for gain adjustments. Audio panels that are not set up and leveled properly can induce noise as well. Hight voltage strobe wiring is or used to be a common culprit. LED systems are pretty quiet now days.
Don't sweat a VSWR around 2 much. Look around the system for the bad guys.... that are letting the smoke out.
And buy a quality belly antenna from DP or Spruce. (no affiliation with either)
Nick
Avionics Tech- 41 years ATP/CFII/HELO/SEA/BALLOON/P25 Tactical

[Doug]

Originally Posted by flightlogic

A modern radio will protect itself from reflected power (which manifests as heat in the final amp transistors). The blowing up phrase is quite misleading.

Glad for your expertise here. Unfortunately I do not get to review the circuits for the modern radios used in aircraft so I cannot contribute to the discussion. My question is: is this true for the MGL V6 radio? The "blowing up" comment from the manufacturer's represantatnive suggest they may not have such protection.

I agree about the helpful descriptions of the antenna designs.

Another factor with belly mounting an antenna on the tailwheel RV is the proximity of the gearlegs to the antenna. These are very colse to the 1/4 wavelength of the antenna itself so will significantly influence the radiation pattern and possibly the VSWR as well.

[flightlogic]

There is an RF designer/engineer who reads our threads but usually does not post.... to avoid employer conflict of interest etc.
I have seen his radio design work close up for over two decades. He lives and breathes this stuff....
I have permission to copy and paste a bit of his knowledge here:

{{{{{{{Bent whip antennas were designed for bottom mount installations and there are no issues with this aviator using a bent whip antenna as he intends to.



The typical aviation antenna, especially a bent whip can present ~ a 2.25:1 VSWR at the band edges in a fixed wing installation in a bottom mount install.



In fact in a helicopter install, which most comm. radios are also designed and marketed to do double duty, the VSWR in either a bottom mount between the skids or for a top mount near the rotor will see a low VSWR around 2.3:1 at midband frequencies and it will see a band edge VSWR around 3:1 or higher which is still acceptable for the transmitter.



Some additional information on comm. radio designs: Many transmitters are also designed to have the sidetone audio directly sampled off of the reflected power of the transmitters output and this is done so that the sidetone audio will increase in amplitude, almost to the point of becoming distorted and this is normal and is used as an indication of approaching a really really high VSWR level, sort of a audible VSWR indication to the user.



And even in these cases the transmitter is still entirely protected and capable of handling the extremely high VSWR.}}}}}}

Regards, Flightlogic