CTHA Antenna

 

 

Since  some time I wanted try the CTHA antenna, so I did look for some infos in Internet.

Not having found any clear information about how design a CTHA for a specific band I decided to build one using the materials available in the shack just to investigate its behaviour so to be able to scale it to some radio amateur band. With a flexible  7cm diameter tube I managed to build a toroid with a diameter of 60cm. On it I did wound 2 helix of 15 loop counter-wound.

I did put the CTHA on a plastic table in the garden connected with a RG58 cable and I did start to measure SWR using as source  my IC-706 MKII at 10W and the SWR meter Revex W570.

 

I want to share the results of my investigation with colleagues OM as what I have found is really astonishing.

I was looking for three resonances as described in the article “A Study of the CTHA Based on Analytical Models” by Douglas B. Miron, but with my surprise I found a lot of resonances with SWR exactly equal 1. Here is the list:

 

23.920

27.510

31.115

35.215

39.675

43.105

46.560

51.970

55.200

58.600

70.220

72.020

74.380

77.320

81.460

84.040

85.920

88.380

…… the IC706 do not transmit much in FM band

139.280

146.360

 

And maybe others.

Besides I noted that SWR is <=2 in the range 23.960 MHz to 8.880 MHz.

 

 I wanted check if it is possible to tune the CTHA to a different frequency so I did connect a small variable capacitor between the cross of the two helices in the position just diametrically opposite to the feeding point. I have found that the resonances change their values. Here is a short list limited up to 60 MHz.

 

17.150

19.025   SWR=1.2

23.865

27.295

31.250

35.600

40.305

43.800

47.615

50.800

54.320

57.825   SWR=1.5

 

 Where not stated SWR is exactly 1 !!

In the zone 15.940 MHz to 20.985 MHz the SWR is <= 1.5 with oscillations.

 

The resonances are quit wide: just an example: at 27.295 MHz the SWR is <= 1.5 between 26.590MHz and 28.220 MHz, actually a band usable of 1.630MHz !!

 

Just for fun I did replace the capacitor with a short-circuit. Not a great change ! here is the list of resonances with the short-circuit.

 

16.950

18.935

19.115

23.895

27.395

31.265

35.660

40.325

43.880

47.695

50.810

54.230

57.915    SWR=1.4

 

I had at hand an RG58 cable of 1.6m with connectors so I did insert it to make the total cable a little bit longer. Living the short-circuit in place here is the list of the new resonances:

 

22.615

25.690

29.395

32.790

36.790

40.780

44.000

47.015

49.715

52.280

55.430

58.560

 

Being SWR <=2 between 11.850MHz and 24.025MHz

 

It seems that the cable is participating to the value of resonances, but SWR is always just 1 !! this did puzzle me very much.

 

I have lifted the antenna on a wooden post and put it in vertical position getting some frequency shift of resonances.

 

I tried to do some numerology to correlate resonances with the antenna measures, but without any success.

 

A mad idea was tormenting me: I had the first time this idea when adjusting the 40m magnetic loop shown elsewhere on this site.

When used in the exact resonance frequency the antenna is “willing to transmit” with violence !!

I was using a power supply for the IC-706 made of two switching PC power supplies paralleled and modified to give 13.8V. The magnetic loop was un the roof of my pick-up and the IC-706 and the power supply was un the floor at about 5 meters. When sweeping the tuning knob of the transmitter, passing exactly on the frequency of resonance 7.060MHz, the emission was so violent to crash the power supply; I had to put another power supply old stile to go ahead in tests. 

 

Coming back to the CTHA, I was thinking that at the resonances the antenna is so willing to transmit that, whichever is the Z of the cable, the reflected power is ZERO because it is all transmitted, giving an SWR exactly 1.

 

To verify this mad idea I did insert between the CTHA and the feeding cable a BALUN 4:1 that I had at hand, previously used for a folded dipole.

The idea was that if the impedance of the CTHA is 50 Ohm, putting the BALUN 4:1 the SWR should rise to 4 at least.

 

My mad idea has been verified !!! SWR at resonances is still 1 !! Here is the list of frequencies a little bit moved. (Short-circuit on the opposite side in place)

 

18.880

24.660

28.185

31.765

35.900

40.290

43.935

47.535

50.595

53.545

59.510

 

With SWR <=2 between 10.880MHz and 22.955MHz.

Further more now I have the following SWR in amateur bands:

 

20m   14.000   1.8

14.350        1.7

 

17m   18.068    1.4

18.168          1.4

 

15m    21.000   1.4

           21.450   1.2

 

12m    24.880   1.1

24.990        1.2

 

10m    28.000   1.0

28.500        1.2

29.0                 1.5

 

6m     50.000   1.3

54.0           1.2  with two minima in the middle.

 

Isn’t it astonishing ???

Well I do not know if it is an antenna or a dummy load but at least I do not need an antenna tuner !

 

Before putting the 1:4 BALUN I made a QSO with LU3DDH at 28.330MHz, but propagation was very bad.

I have got the idea that the best efficiency is between 70 and 50 MHz but I am not sure of this.

I nice test would be the following:

 

Put two equal antennas 100m apart, one with a TX and the other with an RX and plot the received field in function of the frequency. With such a setup should be evident where the antennas work better even if it is not possible to measure the real efficiency. I am sorry because I can not do this experiment because I do not have a second wideband RX an a friend to help me.

 

One more observation:

During the first day of tests, with the antenna horizontal at 70cm over ground on a plastic table  I happened to tune at 52.765MHz a sub-harmonic of a local FM station (Radio San Pedro FM 105.5) at about 3 Km from my QTH. The S-meter was giving 9+60dB.

The following day, when the antenna was vertical tied to a pole, I wanted listen to some music, so tuned the RX to 52.765MHz…..but nothing at all !!

“Strange”,  I thought, “maybe it is not transmitting”….so just by accident I switched to an other antenna and the signal appeared !! S9+++

Never seen before such a big separations between orthogonal polarizations !!!!

 

So, I hope that this short note will stimulate further studies on this strange antenna.

Why it is bewaring in such a wide band mode ?

Which relation between the physical dimensions and the resonances ?

How to optimize the CTHA for HAM bands ?

Which efficiency may be expected for mobile use on 40m and 80m ?

 

73 de

Pino ZP4KFX    28th July 2006  

 

(modified on 4th November 2006)