DIY 434MHz Antenna

[Bill Harvey]

Hi;

I have bought an RT2832U R820T SDR USB Device from Cozycave and now need to build some suitable Antennas for it. I would like to build a Colinear Antenna for our base station and a Yaggi Antenna to use as a finder for the chase car. I also need a Mag Mounted Antenna for the chase car?

I have been looking at some guides on the net but as I don't know exactly what I should be building I would like some pointers on the guides / specs to ensure build a correctly tuned Antenna.

For HAB tracking I understand that I need an antenna capable of receiving 434MHz.

The cable I have is HDF400 from Solwise (2 Metres) here:
http://www.solwise.co.uk/wireless-cable.htm

The Tech Specs from Solwise only show the attenuation at 2.5Ghz (0.554dB/m) however a quick search for the same cable on Amazon:

http://www.amazon.co.uk/HDF400-Coaxial-Cable-200M-Drum/dp/B00OZQVYO6

Gives attenuation for many uses and shows 450MHz attenuation is 0.089dB/m. The velocity factor for this cable appears to be 85% 0r 0.85 so according to this calculator here:

http://jeroen.steeman.org/Antenna/collinear-coax

My 1/4 wave segments would be 146.79mm long, does this seem OK?

The page on which the calculators appears above also gives a basic example of building a Colinear Antenna using both 1/4 and 1/2 wave segments. I have 2m of cable so I am just short for a 1/2 wave segments antenna (7 segments (2 x 1/4 and 6 x 1/2) I am 5cm short!!) so I will have to make an 8 segment 1/4 wave antenna.

This guide here: http://www.rason.org/Projects/collant/collant.htm

Gives some additional information and suggest adding an additional 1/4 wave length of 16 Gauge Copper to the core of the top segment (simple enough).

In both guides I cannot quite work out however how the bottom segment is joined.

One guide shows a length of coax trailing away but it is confusing how it is joined (looks like centre core to centre core and outer braid to outer braid?) Could I simply subsitute this with and N Connector?

The other Guide doesn't show how the bottom is joined and confuses the issue by suggest a covered element using aluminium or copper solders at the antenna feed point (which is where?)

Some advice please :-)

[Steve Aerospace]

Hi Bill - its great to see people making antennas - so often folks just buy stuff with no clue to how its made or works.

Ive seen many variations of this antenna - but I can’t vouch for any particular one.

A velocity factor of 0.85 seems pretty standard for HDF-400 making a 1/4 wave about 147mm as you say. I wouldn’t get too hung up on sub mm accuracy - cutting to mm accuracy will be difficult enough - plus expect some variation in velocity factor from specified.

The bottom section is joined identically to all the other sections - with the exception that an aluminium sleeve pushed over the co-ax and connected to the outer braiding at the joint. A copper sleeve is also suggested and that might be a bit easier to make a good electrical joint (brass would work too).

The feed point is where the lower length of coax (the feed coax) is connected to the first of the 1/2 (or 1/4) wave sections.

You can place an N connector at any point after the Torroid choke - I.e. a 1/2 wave or more below the feed point.

Ideally you would get the antenna tested with someone with an SWR meter (you will need a 70cms transmitter and ham radio licence) or (better) a VNA.

I hope that helps.

Steve G8KHW

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[Phil]

loss from a suppier.

HDF400 low loss coaxial cable, suitable for WiFi

Nominal Attenuation -
30MHz - 0.022dB/m
50MHz - 0.029dB/m
150MHz - 0.05dB/m
220MHz - 0.061dB/m
450MHz - 0.089dB/m
900MHz - 0.128dB/m
1500MHz - 0.168dB/m
2000MHz - 0.196dB/m
2500MHz - 0.222dB/m
5800MHz - 0.355dB/m
So I would expect it to be the 450Mhz figure as a feeder.
When it is a part of the antenna, it is not a feeder,
The bottom part is a matching stub, so it is the tube and the ferrite rings. then the co-ax is almost any length.
I would house the antenna in a White PVC tube. Black ones normally have carbon in them, and affect the performance.
73
Phil G4EGU

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[Steve Aerospace]

Another comment Bill :- you say “so I will have to make an 8 segment 1/4 wave antenna.” I’ve not seen such a design - the ones I’ve seen have mostly 1/2 wave sections - what is your source for that?

In general be wary of antenna designs on the internet - many have no basis in theory.

Most antenna designs work so some extent or another - I’d be wary of any design that doesn’t back it up with some measurements of feed impedance/SWR and gain.

Steve G8KHW

[Bill Harvey]

The two guides I am looking at are here:
http://www.rason.org/Projects/collant/collant.htm
and here:
http://jeroen.steeman.org/Antenna/collinear-coax

Which I linked in my first post, the first is 1/2 wave length and so is the second but in the second its says that if length is an issue then 1/4 wave length will do fine?

[Bill Harvey]

Just made some calculations and for a 1/2 wavelength antenna allowing for cutting / joining, a feed line and end antenna I'll need just over 3m of coax. (1/2 wavelength segment = 301.58mm, rounded to 302mm x 8 = 2416mm + 155mm End Element + 450mm Feed Element (3/4 wavelength) = 3021mm).
So my plan is to use the 2m coax that I have and make a 1/4 wavelength version and test it out. It will still be nearly 2m (1845mm)

Or build a 1/2 wavelength antenna with less segments and add when I get some more?

[Steve Aerospace]

Hi Bill,
I really don’t know how a 1/4 wave version would work - I’d be dubious of it.

1/2 wave segments work like this:

Each electrically 1/2 wave segment (i.e. Velocity factor x 0.5 wavelength) has the property of:
a) presenting the impedance at one end to the other without transformation
b) delaying the signal by 180 degrees

Thus by alternating the inside and outside connections for each segment:
the impedance presented to the feed point is equivalent to that of the 8 segments in parallel.
the radiation from each segment is in phase with the others (180 + 180 degrees).

Being near 1/2 wave each segment has a high feed impedance - but paralleling 8 up is supposed to bring the impedance down to near the co-ax feed impedance.

1/4 wave segments have the property of transforming the impedance at one end to something completely different at the other. They also delay the signal 90 degrees - so with the alternating connections the segments are -90 degrees out of phase with each other. Co-linears are supposed to have the radiating elements in phase with each other - so I really don’t know how a 1/4 wave version would work.

Steve G8KHW

Steve Randall
Random Engineering Ltd
st...@randomaerospace.com
+44 7802 242135

[Bill Harvey]

Steve;

You clearly know your antenna science and I will order some more coax and make a half wavelength version just to be sure. In the mean time in anticipation of trying to track a couple of flights today that I had spotted advertised on here, I made a half wavelength version last night.

I had trouble with the N connector on the end of the antenna and forgot to check it with a multimeter for shorts but when connected I did managed to pick up an FM station with Gqrx

http://i1043.photobucket.com/albums/b436/JonnyAlpha/Electronics%20and%20Robotics/Screen%20Shot%202016-04-10%20at%2023.15.11_zpsau4aso05.png

I did try and track the HAB flights today (HERMES) but as I am still not really sure how set up Gqrx and dl-fldigi I didn't receive anything, so this is either my antenna or me?

Here is a screenshot of gqrx and dl-fldigi:

http://i1043.photobucket.com/albums/b436/JonnyAlpha/Electronics%20and%20Robotics/Screen%20Shot%202016-04-11%20at%2017.18.47_zpspoq2gzww.png

I am not sure if you have to set the frequency on dl-fldigi as well but you'll notice it is not showing 434.50MHz it was but when I move the cursor using the trackpad on my Mac to do anything after setting the frequency it randomly changes. It was set at 434.50 before taking the screenshot.

[Bill Harvey]

On Saturday, April 9, 2016 at 3:41:59 PM UTC+1, Bill Harvey wrote:

Steve;

Just seen your PM, also just realised I have also been talking to you on GSBC ref CozyCave RTLSDR :-)

[Steve Aerospace]

Yep - I already guessed from the URLs you sent:

http://i1043.photobucket.com/albums/b436/JonnyAlpha   …

:-)

Steve

On 11 Apr 2016, at 22:26, 'Bill Harvey' via UKHAS <uk...@googlegroups.com> wrote:

Steve;

Just seen your PM, also just realised I have also been talking to you on GSBC ref CozyCave RTLSDR :-)

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[Ian Gibbs]

Hi Bill.

I have just built and tested (although not scientifically) http://www.qsl.net/dk7zb/PVC-Yagis/7-Ele-70cm.htm for tracking our payload. It worked very nicely, with a strong signal at 150m from an NTX2B which had no antenna attached at all. It maintained good directionality right up to 20m away, so I feel confident it will serve us well when we come to launch. 

I printed the parts at http://www.thingiverse.com/thing:912567 on my 3D printer. I have the left over brass and aluminium rod and plastic boom tube to build another and can print you the parts if you would like, although not for a couple of weeks as my printer queue is maxed out.

[Stuart Robinson]

On Wednesday, April 13, 2016 at 10:42:42 AM UTC+1, Ian Gibbs wrote:
> Hi Bill.
>
>
> I have just built and tested (although not scientifically) http://www.qsl.net/dk7zb/PVC-Yagis/7-Ele-70cm.htm for tracking our payload. It worked very nicely, with a strong signal at 150m from an NTX2B which had no antenna attached at all. It maintained good directionality right up to 20m away, so I feel confident it will serve us well when we come to launch. 

Its not difficult to add a bit of science and you probably have the equipment already.

As long and the signal from the NTX2 is reduced in some way, so as not to overload the SDR, then the SDR software, SDR# for instance will give you on screen the signal strength in dB.

So with your receiving setup out in the field, just compare the signal strength between a standard vertical antenna, maybe a simple dipole and your home brew yagi. It wont take long to do the comparison, and you will then know the relative gain in dB of the various antennas.

[Ian Gibbs]

I agree. I'm not short on equipment or ability, just time.

[Phil]

not prying,
But if there is a possibility of overload.
You can always add an attenuator, in the RX line.
or use a 3 ele hand held beam, to find the paskage.
Phil
G4EGU

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[Bill Harvey]

On Wednesday, April 13, 2016 at 10:42:42 AM UTC+1, Ian Gibbs wrote:

Did you use the Yagi at your static base station? I was planning on building a Yagi to use as a Mobile Tracker when we get near to the landing site? WOuld this Yaggi be good for that purpose also?