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Feedline loss in EARCHI antenna?


#14

Yes, agreed that random wires work fine with counterpoises and very short (or no) feedlines.

I’m pretty sure he measured those SWRs at the end of 25ft of coax. I agree with you that you need to measure SWR at the matchbox. The numbers at the end of the coax are almost meaningless unless you do the Smith Chart reverse calculation etc. If I get inspired, perhaps I’ll do some measurements at the matchbox.

73, Barry N1EU


#15

I put out a ~53ft wire (26ga Sotabeams wire)… ~45Deg angle into a tree with a Balun Designs 9:1 and a 12" piece of RG8x into a Rig Expert…

SWR Dips of …
1.2
1.7
1.9
3.x

This proves my wire isn’t the correct length… :wink:

I also pushed the wire directly into the connector on the Rig expert… it gave me an SWR of 17.5:1 at ~15,255Mhz

Richard // N2GBR


#16

I think that sweep shows a 10:1 SWR in the ham bands, correct?


#17

Barry.
I got this https://www.ebay.ca/itm/QRP-Backpacking-Multi-band-End-fed-Antenna-System-With-Coax-SOTA-POTA-/332357583357?hash=item4d620e41fd last spring to go with my KX2. It tuned it but not very many contacts maybe due to the 31’ of wire. I’m about to make my own 9:1 unun shortly and plan to try and run it with at least 58’.
Here is more info on them. http://www.hamuniverse.com/randomwireantennalengths.html
I use 25’ of RG-58 instead of what most use (RG-174) so not sure where that is going to fit in.

Malen
VE6VID


#18

Malen, if you are going to try and use this type of antenna, wind the transformer with a type 61 ferrite core (8 turns). Some of the links above discussed why this is the best choice.


#19

I’m not sure why activators would want to carry a long coax feedline, counterpoise wire, etc. I realize the guys that want to put up dipoles need to do that. I’m glad I abandoned dipoles and now use an EFHW. My feedline is really short and the only reason I have that is to have more flexibility in my operating position on the high summits. I’ve always used resonant antennas as I didn’t want to carry an external tuner. I now have a KX2 with internal tuner. I can now tune my existing EFHW to more bands without making a new antenna ;-).

Sorry for the brief sidebar Barry! Back to the topic…

73, Brad
WA6MM


#20

Removing the 50ohm coax…
I didn’t have the correct connectors to go from the matching 9:1 unun to the Rig Expert directly so I had to McGyver it…

anyway…

Richard // N2GBR


#21

Barry,
I have T130-2 cores sitting here that I was going to use. I have read that it is workable as well.

edit to add: I just dug out a FT140-43. Would I be wise to use it instead?

Malen


#22

The plots show resonance at the harmonics of the fundamental (7.6275MHz) and SWR well above 10:1 on the ham bands.


#23

Yes, and if you can get the Rig Expert to give you an SWR across the range its actually above 17:1…

Richard // N2GBR


#24

Thanks for the legwork Richard! So basically you’re looking at 5dB of feedline loss if you use the 25ft RG174 as that vendor suggested. You’ll make plenty of contacts but you’re going to lose the weaker paths. As Brad suggested, if you’re going to use one of these, you should keep the feedline length to a minimum and use coax thicker than RG174 if possible.


#25

I have rolled my own 9:1 on a type 61 core and it works quite well, using 58’ has given the best performance but will work with 41 as well. I use a counterpoise wire ~ 12’ and deploy with a 30’ pole. Feedline is usually 30’ or RG-8X. Pre tuner the SWR measured at end of feedline has been as high as 6 or 7 to 1. Easily handled by my LDG 817 tuner.

With regard to RG-174…feh! Better off plugging the wires directly into the tuner ala the BNC to Binding Post adapter…no feedline loss.


#26

KE6MAK has got this right, the feedline and unun are pointless junk if you still need to use an atu. Choose a wire length that avoids the impedance extremes on the bands you want operate, throw out between 2 and 4 radials on the ground and plug straight in. As long as you are not matching an antenna that is way too short the losses will be low.

With my KX2 I use 4 x radials 7.5m long
6m travel pole
40/30/20m 9m wire, inverted L 6m up and 3m out
30/20/17m 7.2m wire wrapped round the pole
17m - 10m 4.5m wire vertical

These lengths all range between just under a quarter wave to just under a half wave so avoid the impedance extremes and give low angle radiation. The vertical is a good performer for SOTA, even with very poor ground and limited radials. The HB9SOTA antenna tests demonstrated this in the tests below.

Horizontal antennas do have ground reflection gain, but this is all at too high an angle to be useful on 40m and above. 40m doesnt provide NVIS at higher latitudes except around solar maxima, so no help to most of us currently.You may of course get low angle gain from a low dipole, but you need to be very near steeply sloping gound. You can model this using HFTA which comes with the later ARRL Antenna handbooks, my conclusions were you need to be on the edge of the sloping ground to get a significant advantage.

73 Gavin
GM0GAV


#27

One important detail to note (not explained in the PDF) is that in this test, the vertical wire coming down from the pole was directly connected to the KX3, without any coax whatsoever. I think that is the reason why it performed so well, but may not always be convenient depending on the summit layout and whether one wants to guy the pole or attach it to some existing structure. Perhaps this is where the T1 comes in handy, even if one already has an ATU in the rig?


#28

This is not necessarily true.

In my case I bought an M0CVO LW-20 antenna for holiday use. This is a 20 metre long wire with a 9:1 unun. I tried it out in my garden. Now due to the very high noise level in my urban location when I installed it close to the house, I re-installed it with the unun halfway down the garden, about 25 metres from the house, which meant using a long run of coax to my tuner. This is why I said that the comment was not necessarily true, a long run of coax enabled me to use the inverse square law to reduce the electronic smog that I have to live with!:grinning:

Actually the antenna worked surprisingly well, giving results quite comparable with my usual antenna, a 102-foot doublet that started life as a commercial G5RV - well it is still a G5RV as its originator preferred to use it as a doublet himself! After a trial over several weeks I removed it and re-installed the G5RV, the LW-20 is now packed away for holiday use.

Feedline loss was indubitably present with such a long run of coax, and of course it would be less with the 25ft of RG174 of the EARCHI antenna. I note that the data sheet for RG174 gives a loss of 8.9dB per 100ft at 100 MHz, with an operating voltage of 1500 volts. On the HF bands I would expect the losses to be significantly less than 8.9dB/100ft. It seems to me that 25ft of coax seems an oddly specific length, I guess that this length is integral to the operation of the EARCHI antenna.


#29

With 25ft of RG-174 at 28.5MHz the cable loss is about 1.1 dB which is acceptable even for us at QRP levels. However, that is at 1:1 SWR, move that to 10:1 and the loss is 3.6dB which is not really acceptable. So a short (1m or 3ft) of coax from the radio to the unun is not going to really be an issue, but 25ft maybe if the SWR seen at the input to the unun is large… and it can be with a random wire + unun.

As I am a bear of little brain I would rather have an end-fed with links or traps if I wanted a “one-wire” antenna (and maybe a short counterpoise to mop-up location variables). I’d know that the antenna should look like 50ohms on the bands I want to use so any excessive SWR indication would tell me something was in error.


#30

Since an end-fed, whether with links or traps, would need matching to the rig, I think a W3EDP with a simple parallel tuner is more convenient for SOTA.


#31

Well I was assuming we’d understand that an end-fed (really an EFHW) would need an impedance transformer at the feed point. The serious point being a link/trapped EFHW would present the same very high impedance at resonance for each band selected meaning the transformer would be a fixed, no-tune device. In that case the trapped version would be the install and use antenna requiring no tuning in situ and the linked version would need you to get up and change the links. But neither would need a tuner in the classical sense. I suppose I should have made it clear I was thinking of an EFHW.


#32

Of course connecting the radiator straight to the rig may not suit every situation. I normally leave the bottom of the radiator wire free with enough slack so that I can sit next the pole with the rig on my knee. See the picture. It makes little difference to the pattern having 1m of wire at the bottom going horizontal or at an angle.

As G8ADD points out this would not be good for entering a shack or building, or where you have local electrical noise beside you. Thats where the remote auto ATU is the best solution, rather than running coax with a bad mismatch.

For SOTA when you already have a atu in the circuit, what value does the coax + unun add?

73 Gavin
GM0GAV


#33

I use a random wire on 62 feet and and 72 feet with tuner and a one to one balun can use all bands from 80 to 6 Mtrs 62 has trouble with 60 if that’s of any use