Which VHF Com Antenna and NanoVNA testing SWR & Impedance

I have 2 VHF Com antennas, bent whips. Which one would you pick. See Pic links.

Dorne and Margolin Part# DM C63-2 Fiberglas
Decibel Products Inc. Part# 870 ??15A, Stainless. (?? not clear)

DP Inc. Stainless is 8.6 Oz. and a little more a mystery. It is stout, potted, machined aluminum base. The mystery is the markings are minimal. DP Inc. is a big Tel-Com antenna company but seems to be out of aircraft antennas.

DM Fiberglass is 6.2 Oz. has label with serial number and STC number. It is nicely made but not the brick house the DP Inc. is.

https://drive.google.com/file/d/1XzAvbHsOS6E0Ratxc1X5uS4L2cqw9aBj/view?usp=drivesdk

https://drive.google.com/file/d/1Y-lHsP6eJjh5HxynJNYAG7jGL9Lq15Zz/view?usp=drivesdk

https://drive.google.com/file/d/1Y4cHUmBd7MHLWR-AzJzGNuKCn6WXoL1V/view?usp=drivesdk

https://drive.google.com/file/d/1Y2RhfOBUJ_FvXfaa18xEMZprKZxUOC3E/view?usp=drivesdk

Testing with NanoVNA is a little tricky, calibration and test set up. I got wild variations depending on test setup. However the crude set up with scrap aluminum seems reasonable. Far from ideal 4 foot Dia. ground plane (full 1/4 wave for monopole antenna about 24 inches). it was better than holding antenna base in hand. SWR and Impedance is always function of Installation, coaxial cable, connectors.

My planned location, belly, off center forward of spar. Not ideal but easy Installation and short coax. TW gear I'm sure that will influence signal directionally. Transponder antenna will be less than 3 feet away. There is no perfect installation and small planes have limited and no ideal unobstructed ground-planes. Good enough is the key. Top aft fuselage half way between aft cockpit and vertical Stab seems better at least air to air. Belly antennas (aft fuselage) is better for air to ground but subject to damage if too far aft on our small RV's.

Overall the better one is DP Inc. Weight is more but only 2.4 Oz. The Impedance is always more stable and overall test SWR is better or slightly higher than DM. However 1.5 SWR or slightly above is still a win. The DM glass Antenna does better at 118 Mhz end than DP Inc., but the DP Inc. is better Mid band. The biggest RF factor ( knowing my data may be wonky) is the DP Decibel Product Inc always to seem to have better Impedance match across the band.

Really can't tell which one is better RF performance until installed with RG400. Best is after full installation, key the transmitter with an inline SWR / Power meter. By guess is no way to tell the difference in the real world application, may be a lab. The DP Inc. I longer, but DM may use some electrical tricks make it appear longer (e.g. matching). The impedance on the DM seems to be further off but again could be me or could be some matching network inside the antenna?
Arguably the COM radio is one of the most important pieces of equip in the aircraft.

What is your opinion? Fiberglass or Stainless? Installation do's and don'ts (horse beat to death but let's discuss if you have insight.

The COM1 straight top antenna works fine.
My COM2 bent whip belly antenna works fine in the air.
However on ground at large airports with uneven surface it does not work
very well.

Good luck

Avanza said:

The COM1 straight top antenna works fine.
My COM2 bent whip belly antenna works fine in the air.
However on ground at large airports with uneven surface it does not work
very well.

Good luck

Click to expand...

I’ve had two occasions in a dozen years where my belly antenna would not communicate with ground. Both times, a 30 deg turn fixed the problem. So yes, not as good for ground ops, but in my experience good enough - usually!
If it were my choice, I’d go with the D&M. ‘Short’ antennas use internal components to get a good impedance match. As they age, the match may not be so good. Also, since these components are frequency dependent, you usually end up with a poorer match at the two ends of the frequency band. Finally, the shorter antenna will radiate a bit less power due to it being short (but only a bit less, as the majority of the radiated power comes from the part near the base).