Simple but effective DIY Civil and Military Airband Antenna

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merseyradar
Posts: 3
Joined: Mon Feb 24, 2020 12:15 am

Simple but effective DIY Civil and Military Airband Antenna

Post by merseyradar » Mon Feb 24, 2020 12:22 am

Hi rtl-sdr hobby community,

Double Bazooka Coaxial Dipole made specifically for VHF/UHF airband.

Image

Nice DIY hobby project for Civil/Military airband monitoring fans.
Please take a look at my blog page linked below for construction details & photo guide, an inexpensive project, not too difficult to make with very good performance if you build it right. Great feedback received from builders all over the world... :D

http://www.merseyradar.co.uk/airband-ra ... l-airband/

Mike

http://www.merseyradar.co.uk/

HighSNR
Posts: 51
Joined: Mon Sep 18, 2017 8:14 am

Re: Simple but effective DIY Civil and Military Airband Antenna

Post by HighSNR » Mon Feb 24, 2020 6:26 am

Hi Mike,

Great writeup. I haven't seen this style of antenna before. It's a very interesting design.

The only thing missing from your blog is any final measurements of the antenna performance. The frequency is right in the sweet spot for a £50 nanoVNA. The S11(vswr) will give you an excellent idea of how close to optimal this antenna really is.

HexAndFlex

snn47
Posts: 213
Joined: Tue Dec 27, 2016 11:00 pm

Re: Simple but not broadband Antenna

Post by snn47 » Mon Feb 24, 2020 8:28 am

The only thing missing from your blog is any final measurements of the antenna performance. The frequency is right in the sweet spot for a £50 nanoVNA. The S11(vswr) will give you an excellent idea of how close to optimal this antenna really is.
Good antenna performancance is not just having a low VSWR. A 50 Ohm Dummy Load is broadband and will provide nearly no return loss if meassured, however it is not a very efficient antenna.

I would be surprised if the antenna design would cover 118 to 400 MHz as the title suggests, since the variation in wavelength is ~2.54 to 0.75 m wavelength variation factor 3.4!

This antenna type was described by W8TV(QST http://www.w1npp.org/events/2010/2010-F ... 0703~2.PDF) for use HAM shortwave Bands which have a much smaller wavelength variation e.g. 3.5 to 4 MHz the wavelength variation factor is just 1,14.

HighSNR
Posts: 51
Joined: Mon Sep 18, 2017 8:14 am

Re: Simple but effective DIY Civil and Military Airband Antenna

Post by HighSNR » Mon Feb 24, 2020 9:35 am

There isn't much material in this build that is likely to add loss(at this frequency) to the antenna, so in this case, I would expect S11 to be a fairly good proxy for the true performance.

merseyradar
Posts: 3
Joined: Mon Feb 24, 2020 12:15 am

Re: Simple but not broadband Antenna

Post by merseyradar » Mon Feb 24, 2020 12:32 pm

snn47 wrote:
Mon Feb 24, 2020 8:28 am

I would be surprised if the antenna design would cover 118 to 400 MHz as the title suggests, since the variation in wavelength is ~2.54 to 0.75 m wavelength variation factor 3.4!

Hi snn47,

There are 2 antenna designs in the article, one for Civil Airband which has 1.17mtrs/3ft 10 inches overall length (128mhz centre freq), and one for Military Airband which is around 0.5mtrs / 1ft 8 inches overall length (300mhz centre freq).

I do mention in the article that the Double Bazooka design was featured in July 1968 QST Magazine which you have linked to.

Hi HighSNR

I dont have a mini vna at my disposal but I may invest in one. The cheap models are interesting because there is always improvements to the firmware and builds. Seems like a very popular hobbyist item so updates/improvements are mostly coming from a keen user/enthusiast community. One thing to consider is I see lots of people relying on VNA readings when testing antennas "on the bench" where the test signal could be affected by surrounding materials/people and not in the true operating position and in free air.

I did own an AW07A antenna analyser for a short period but that was not very good at all, in fact a right royal PITA !
My experience with that (and the supplier) is detailed below

http://www.merseyradar.co.uk/airband-ra ... rom-radio/


Regards,
Mike

http://www.merseyradar.co.uk/

snn47
Posts: 213
Joined: Tue Dec 27, 2016 11:00 pm

Low VSWR is not an indication for good antenna performance!

Post by snn47 » Tue Feb 25, 2020 5:00 am

@HighSNR
There isn't much material in this build that is likely to add loss(at this frequency) to the antenna, so in this case, I would expect S11 to be a fairly good proxy for the true performance.
You seem to confuse a lot of factors that define good antenna performance.

VSWR is defined by the ratio of forward to reflected power, not loss in an antenna due to resitance of the conductive materials used.

Unless you meassure VSWR directly at the antenna input, the attenuation of the cable between antenna and transceiver/receiver input will lower the VSR because it attenuates the reflected power. High loss cable can provide you with VSWR below 1.5 but will also attenuate signals very much, hence VSWR is not any measure of antenna performance or of antenna efficiency.

Then there is the antenna pattern, unless the antenna has sufficient gain at elevation angles and directions where the signals you want to receive originate from. even an efficient antenna will not provide good performce for the signals you want to receive.

Antenna may have a complex impedance and exhibit bad VSWR but still provide good performance, if resonant at the operational frequency range.

@ merseyradar

I agree the antenna could be wide enough for VHF use, (HAM's use it only between 144 to 148 MHz)if you use RG-213 or thicker cable, but not for UHF.

merseyradar
Posts: 3
Joined: Mon Feb 24, 2020 12:15 am

Re: Simple but effective DIY Civil and Military Airband Antenna

Post by merseyradar » Tue Feb 25, 2020 7:51 pm

Im new here, I respect all forum members views and opinions but this is the only forum I have visited where contributors can change/edit the name of the original thread.

snn47
Posts: 213
Joined: Tue Dec 27, 2016 11:00 pm

Re: Simple but effective DIY Civil and Military Airband Antenna

Post by snn47 » Wed Feb 26, 2020 4:56 am

Welcome to the forum merseyradar.
I am not aware of any froum where anyone, except the initial author (and on some forums also not), can change the title of the thread. However beeing able to create your own Subject-Title when you post a reply is standard operational practice in forums I participate, maybee you havn't tried to edit a Subject-Title so far.

rtlsdrblog
Site Admin
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Re: Simple but effective DIY Civil and Military Airband Antenna

Post by rtlsdrblog » Wed Feb 26, 2020 5:13 am

merseyradar wrote:
Tue Feb 25, 2020 7:51 pm
Im new here, I respect all forum members views and opinions but this is the only forum I have visited where contributors can change/edit the name of the original thread.
The title of the entire thread was not changed by snn47. He only created a sub-thread with a new title.

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