Collinear antenna - shorted at the top or not ?


I am about to start building my own collinear antenna - but checking pages around the net, it seems like there are multiple ways of “ending” the top of the antenna. Not all of them are for receiving ADS-B, but it should be possible to calculate the correct segment lenghts for the ADS-B frequency…

The one at uses a “pigtail” - an exposed innercore at the top, length 1/4 wave

Another at … linear.htm uses a shorted 1/4 wave at the top - but with an outer “sleeve” extending the top element to 1/2 wave …

Any comments on what is the best (or right) way to do it ?


You MUST be sure of the velocity factor of your coax before you start building or you will never get a coax collinear to work successfully.

I don’t short the top or have any “sleeves”.

[Edit] My coax has VF of 0.66 so my element lengths are 91mm each.

Have fun :slight_smile:

Yes, I know … That is why I need to go by a shop one of the next days to get some cable, where the vendor has published information about the velocity factor…

VF 0.66 - isn’t that a bit low? I usually see 0.83-0.86 on the cable types I have been investigating.

What kind of cable do you use - 50 or 75 ohm impedance - RG6, RG11, RG58, RG59 LMR200 or similar ?

Whatever DIY antenna you make, to know how good or bad it is, it is advisable to compare it’s performance with a reference or benchmark antenna. The comparison should be done with following conditions:

(1) Both the “antenna under test” and the “bench mark antenna” are at same location.
(2) Same or identical hardware (feeder coax type & length, receiver) for both antennas.
(3) Preferably without amplifier (built-in or external).
(4) Same gain settings of dump1090/receiver for both antennas.

If you already don’t have a good antenna for use as benchmark, you can make a 1/4 wavelength ground plane antenna (Spider or Cantenna) to serve as benchmark. These are easy to make & sure antennas and can serve as good benchmark.

Good luck with your DIY CoCo.
Please do post your results when you finish it and put it to test/service.

For most makes of Polyethylene (PE) insulated cables, VF is in the range of 0.66

For most makes of Foamed Polyethylene (FPE or PEF) insulated cables, VF is in the range of 0.82 to 0.86 depending on degree of foaming (amount of atomized air bubbles injected into molten Polyethylene - higher the amount of air, higher the VF).

Actually, I have been considered getting one of these:

And then put a N->SMA converter on it, and get a 15M LMR-195/CFD200 cable to transport the signal to my receiver - which I consider to upgrade to a FlightAware Pro stick…

The question is just if any amplification will be needed of the signal. From what I can see, it looks like the 15M of LMR 195 cable will give an attenuation of approx 6-10dB - and I am a bit limited on space with the antenna, as the local rules for the buildings and so on states, that antennas not must be visible 1.80M above ground - so I will have to hide it inside my fibreglass flagpole (and I am not sure I can get a weatherproof amplifier that will fit in there - for a reasonable cost)…

I’ve had one of those for years - have a look at my stats

I have at a height of about 9m / 27 feet - just above chimney height

connected to about 8m of LMR400 and a FlightAware ProPlus stick (upgraded from the Pro Stick a couple of days ago)

Use a better cable with lower loss, proPlus stick - no amp up the mast.

I just need to find a place where I can get the cable at a reasonable price (Germany/UK - with shipping to Denmark) - I probably need to mount one of the connectors on the cable myself, in order to get the signal into the house (without having to drill a big hole for a N-connector)…

So far most search results on a 15M LMR400 cable ends up with a cable from Cisco - priced above €200,- :neutral_face:

Maybe a RG213 cable can be used instead ? It is easier to find…

Edit: Found a regional dealer with RF-400-UF cable - attenuation 13.3dB/100m - then I will just have to solder N connectors on myself…

I got mine from (actually I got HDF400, LMR is a branded name this is a clone)

see … 1813044796 … 2108522266

The 50m drum is double your need - maybe someone else there will share the cost

I’m sure there are other cables that will work well

RG6 has a loss of 6.1 db per 100ft / 30m - it’s a lot cheaper
LMR400 4.3db

What do your satellite installers use for domestic dish down leads if they need a long wire?

Probably the same as RF400 here:

Actually, I am not sure… Probably something like Triax KOKA 500 or similar. Usually cable running between the dish and the receiver can be kept below 20 meters…

A related question - when specifying the height of the antenna - is it the bottom, middle or top of the antenna (imagine an antenna 2 meter high, where the bottom if 6 meters above ground level)? Will it be 6, 7 or 8 meters added to the ground level above sea level ?

To the base of the aerial.

For mlat it really doesn’t matter, the measurement errors elsewhere in the system are 50x larger.

Yes, it seems so.

In what stage is your collinear project?

Stage: Acquire the right cable to be used for the collinear antenna :slight_smile:

i would also recommend that you solder the sections together, from my experience just pushing them together doesn’t work that well or if it did work well the moment you move it, it wont.

This i feel this is where a lot of people don’t have any luck with them. I luckily have access to a VNA so trust me on this i can have a perfect coco put it down pick it up again and the turning has changed a lot whilst a soldered one good or bad stays pretty much the same. Heat shrinking the joints with heat shrink with a glue in it will probably help a lot if if your not soldering.

This is why I was considering the principle from … linear.htm - where the shielding of the cable is replaced with brass tube/pipes… It makes it a bit easier to solder things together…

I also need to find a good way to get the cable inside the house, like this one, through a wall outlet like this one:

I most likely have to create my own, using a bulkhead connector like:

I assume, that most types of connectors can be used - N, TNC or SMA …

Don’t use RG213, it’s not shielded enough for f>400 MHz. RG214 is OK, but max 10 meters if you want good performance.

Solder? Try to borrow a crimp tool, or buy connectors that use threading/compression and are mounted with spanners. Soldering should preferably be avoided.

Yes. Don’t put SMA connectors on a >6mm cable, or rigid adapters. The weight and rigidity of the cable can break the SMA-f on your receivers. Use a flexible pigtail to go from N or TNC to SMA or MCX.

TNC is a very nice connector, smaller than N but same performance at 1GHz. Just don’t mix up normal TNC with RP-TNC used with some external Wifi antennas.

Where in Denmark are you located ? I can recommend the “Dansk ADSB” group: